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Micromorphology of opaline features in soils on the sediments of the ex-Lago de Texcoco, México
Geoderma 132 (2006) 89 – 104 www.elsevier.com/locate/geoderma
Micromorphology of opaline features in soils on the sediments of the ex-Lago de Texcoco, Me´xico Ma del Carmen Gutie´rrez-Castorenaa,*, G. Stoopsb, C.A. Ortiz-Solorioa, P. Sa´nchez-Guzma´na a
Soil Science Programme, Colegio de Postgraduados, Montecillo, Texcoco 56230, Me´xico Laboratorium voor Mineralogie, Petrologie en Micropedologie, Ghent University, Krijgslaan 281, S8, 9000 Gent, Belgium
b
Received 5 November 2003; received in revised form 29 April 2005; accepted 4 May 2005 Available online 14 June 2005
Abstract The former Texcoco Lake (ex-Lago de Texcoco) is situated in the closed Basin in Mexico. The lake was artificially drained in the 20th century. Six soil profiles on the eastern border of the former lake were analysed for mineralogical and microfabric characteristics in order to differentiate between sedimentary and pedogenic processes. Large quantities of silica liberated as a result of weathering of the surrounding volcanic mountains partly precipitated in the lake. As a result, layers of an exceptional sediment, locally called bjaboncilloQ, were deposited. They are characterised by a high Si-content and a very high (up to 500%) water retention. In addition several amorphous silica features were formed in these lacustrine conditions. In the centre of the lake opaline nodules, mainly composed of opal-A, formed in the jaboncillo. Near the former coast line concentric silico-calcitic nodules (ooliths) were formed as a result of fluctuations of the input of fresh water and biological activity. Several superposed layers of in situ silicified roots point to frequent changes in the shore line position. After emergence of the sediments, about 100 years ago, a translocation of silica, mainly caused by an alkalisation of the sediment, gave rise to the formation of different pedogenic types of opaline coatings and infillings, resulting locally in a duripan. The micromorphological and mineralogical properties of these silica features are discussed and sedimentary and pedogenic processes considered. D 2005 Elsevier B.V. All rights reserved. Keywords: Jaboncillo; Nodules; Ooliths; Silicified roots
1. Introduction * Corresponding author. Programa de Edafologı´a, Colegio de Postgraduados Montecillo, Texcoco 56230, Me´xico. Tel.: +52 595 95 1 14 74; fax: +52 595 95 1 01 98. E-mail address: [email protected] (M.C. Gutie´rrez-Castorena). 0016-7061/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2005.05.002
Mexico City was built in the Texcoco Lake and since prehispanic times subject to frequent flooding. In the beginning of the 20th century the lake was artifi-
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cially drained. In the strongly alkaline sediments of the former Texcoco Lake the presence of amorphous siliceous materials, mainly of inorganic origin, has been reported, yet not described or discussed (Del Valle, 1983; Gutie´rrez Castorena, 1997; Segura et al., 2000; Gutie´rrez-Castorena et al., 2005). One of these materials is a buried lacustrine sediment layer, locally called bjaboncilloQ, up to 70 m thick and extending over an area of approximately 400 km2 (Departamento del Distrito Federal, 1967–1975). As these materials emerged following drainage about 100 years ago, the sedimentary processes leave have an important imprint on the profile morphology, and pedogenic processes are only in an incipient phase (Gutie´rrez Castorena, 1997). Siliceous features occurring in these sediments, or related to them and formed by sedimentary, biological or pedogenic processes, have not yet been discussed in literature. The objective of the present study is to investigate the relationship of the opaline features with their past and present position in the landscape of the former Texcoco Lake, to distinguish between sedimentary and pedogenic features and to determine the processes involved in their formation.
2. Materials and methods 2.1. Location The study was carried out in the community of Atenco, a site considered as part of the border of the ancient Texcoco Lake in prehispanic times. It is situated in the Mexico Basin, at 2,289 m above sea level (Fig. 1). The area consists of Quaternary lacustrine and alluvial sediments of volcanic origin (Mooser, 1963) and is surrounded by mountains composed of volcanic rocks such as vitreous tuffs (tepetates), rhyolites and andesites. The climate is semi-arid BS KW (W)(i) (Garcı´a, 1968) with a mean annual precipitation of 600 mm and a mean annual temperature of 18 8C. The soils are classified as Petrocalcic Calciustolls (Gutie´rrez Castorena, 1997) and are mainly used for dry land farming. 2.2. Fieldwork Based on a survey, six soil profiles were selected: four on a 400 m long sequence on the Tepetzingo hill, and two on the shore of the ancient lake, close to the village of Atenco (Fig. 2). The Tepetzingo hill, which 1
0
2
3 Km
19°35’
N
Tepetzingo hill
Atenco Village
Huatepec hill
Texcoco Village Ex lago de Texcoco
Mexico State 98°57’ Mexico City
19°30’ 98°53’ Sequence line of soil profiles Border of ex lago de Texcoco during prehispanic times
Fig. 1. Location of the study area.
M.C. Gutie´rrez-Castorena et al. / Geoderma 132 (2006) 89–104
91
Tepetzingo Hill
7 20 m
4 6
10
Alluvial and lacustrine sediments Basaltic rock
14 Lacustrine sediments
17
Atenco Village
Prehispanic ruins
400 m
3 km
Fig. 2. Location of the profiles on Tepetzingo hill (left) and near Atenco village (right).
now rises 10 m above the plain, is an ancient basaltic volcanic island partially covered by alluvial and lacustrine materials. It bears a relict cactaceous vegetation. Each soil horizon or layer was sampled for physical and chemical analyses and oriented, undisturbed samples were obtained for micromorphological studies.
studied by X-ray diffraction after the oxidation of organic matter. Carbonates and iron oxides were analysed in the samples and, they were removed with acetate buffer solution 1 M, dithionite and acid oxalate prior to the phyllosilicate clay analysis (Van Reeuwijk, 1995). No saturations with cations or glycol were made.
2.3. Analyses 3. Results Grain size distribution was determined by the pipette method, pH water measured at a soil/water ratio of 2 / 1, and CaCO3 by acid neutralisation (Van Reeuwijk, 1995). Silica and aluminum in noncrystalline or poorly crystalline forms were extracted by boiling 500 mg of the b 2 mm fraction in 0.5 N NaOH for 2.5 min (Jackson, 1965). Silica and aluminum were measured by Atomic Absorption Spectroscopy (AAS). The undisturbed samples were air dried prior to impregnation with polyester resin and monostyrene (7 : 1 ratio) and thin sections prepared according to the methods of Murphy (1986). Concepts and terminology of Bullock et al. (1985) and Stoops (2003) were applied for the micromorphological study. Special features were also studied with a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analysis system (EDXRA) and by X-ray diffraction (XRD). The sand fraction (250–500 Am) was separated with bromoform (specific density = 2.89 kg/dm3) into a light and a heavy fraction. On grain mounts, 200 grains were counted for each fraction according to the line counting methods. The clay fraction was
A great variety of opaline features were observed, partly of sedimentary, partly of pedogenic origin and varying according to their position in the landscape. Some of the features can be observed readily in the field, at a macroscale whereas other require a more microscopic observation. The physical, chemical and mineralogical analyses of the studied soils are presented in Tables 1, 2 and 3. The distribution of the opaline features is shown in Fig. 3, and their optical and mineralogical properties in Tables 4 and 5. 3.1. Calcitic-opaline ooliths Oolithic (Profiles 4, 6, 7 and 10) layers, 40 to 150 cm thick, were observed from summit (Profile 7) to the footslope (Profile 10) of the hill (Fig. 3). Ooliths occupy from 5 to 80% (volume) of these layers and are the main constituent of the sand fraction, besides euhedral and subhedral grains of plagioclase, volcanic glass, olivine, hornblende and augite (Table 2). The ooliths are about 1 mm in diameter and appear in thin sections as concentric calcitic nodules or nodules with an alternation of
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Table 1 Selected chemical and physical data Horizon
Depth cm
Grain size distribution %
pH 2 : 1
CaCO3%
SiO2%
Al2O3%
SiO2/Al2O3
Sand
Silt
Clay
0–23 23–44 44–58 58–87 87–130
44.4 40.0 40.2 39.9 23.9
47.2 48.3 49.7 45.2 61.5
8.4 11.7 10.1 14.9 14.6
8.2 8.6 9.4 9.7 9.8
18.1 21.9 17.3 20.7 13.8
13.1 10.0 8.2 10.7 5.5
1.6 0.7 0.7 1.2 1.6
13.8 24.2 18.7 15.1 5.8
Profile 4 Ap 2Ckm 3Ckm 4Ckm 5Ck
0–20 20–42 42–75 75–121 121–148
44.7 39.9 48.6 47.3 62.5
40.7 44.6 35.0 35.3 22.8
14.6 15.5 16.4 17.4 14.7
8.0 9.3 9.2 9.1 8.2
33.3 17.1 29.5 21.0 36.8
13.2 9.0 10.9 15.3 14.2
1.3 0.6 0.6 0.6 0.5
17.0 27.2 30.3 46.2 48.4
Profile 6 Ap Bkqm 2CBk 3CBk 3CBk2
0–25 25–40 40–62 62–102 102–139
42.0 45.6 80.0 35.7 59.7
49.0 40.3 11.5 36.2 27.1
9.0 14.1 8.5 28.1 13.2
8.6 8.6 9.1 9.2 9.3
23.4 43.0 16.2 29.0 34.8
8.9 18.1 17.7 8.9 7.9
0.4 0.7 0.9 0.9 0.9
38.0 44.3 34.6 16.6 14.4
Profile 10 Ap 2CBkm 2CBkqm 3CBk 4CBk 5CBk 5Cqk2 6Ckm
0–16 16–32 32–41 41–64 64–101 101–127 127–142 142–165
50.9 81.9 81.0 91.5 81.9 60.4 61.7 70.1
20.5 13.4 12.8 4.6 12.7 25.2 20.0 16.4
28.6 4.7 6.2 3.9 5.4 14.4 18.3 13.5
7.8 8.0 8.1 8.1 8.0 7.5 7.4 7.4
19.9 23.1 29.6 30.2 31.8 17.2 22.7 22.9
22.2 26.3 15.1 6.3 12.8 Nd 25.5 7.9
2.0 0.2 0.7 0.3 0.4 Nd 0.6 0.6
19.1 199.2 38.8 38.7 50.9 Nd 70.6 23.1
Profile 14 Ap A2 AB 2CBk 3Ck
0–30 30–52 52–93 93–110 110–128
34.1 24.7 20.5 40.0 21.0
47.3 54.5 54.9 46.3 63.4
18.6 20.8 24.6 13.7 15.6
7.9 8.2 7.9 7.5 7.6
16.6 11.2 16.6 28.3 35.5
3.9 7.5 6.9 9.8 17.9
0.3 0.3 0.4 0.9 0.9
19.4 37.8 27.1 18.3 34.2
Profile 17 Ap 2A 3CBkqm 3CBkqm2 4Ck 5Ckm 6Ckm 7Ckm
0–19 19–35 35–58 58–74 74–100 100–125 125–140 140–158
59.6 48.0 67.5 68.6 74.3 80.8 74.2 33.9
31.1 38.7 25.9 27.7 21.8 14.9 20.7 43.0
9.3 13.3 6.6 3.7 3.9 4.3 5.1 23.1
8.5 8.4 8.5 8.5 8.4 8.4 8.4 8.0
16.9 19.1 38.4 32.3 32.4 33.6 36.8 37.4
7.3 8.7 10.8 18.9 12.7 14.4 21.6 21.5
1.4 1.3 1.0 0.6 0.5 0.3 1.3 1.2
8.5 11.0 30.7 54.1 40.6 72.5 29.3 30.1
Profile 7 A 2Ck 3Ck 4Ck 5CkR
Nd: not determined.
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Table 2 Mineralogical analysis of sand (250–500 Am) and clay fraction (b2 Am) Depth cm
Heavy minerals
Light minerals
Clay minerals
Op
Ol
Au
Ho
Vg
Va
Qz
Fd
Am
Cr
Fd
Ca
He
Fe
Profile 7 0–23 23–44 44–58 58–87 87–130
1 2 1 1 0
9 17 7 8 7
2 1 2 2 3
5 11 5 5 7
39 23 53 43 25
21 26 8 17 15
5 5 5 7 3
17 24 19 16 39
x x x x xx
– – – x xx
xx xx x x xx
xxx xxx xxx xxx xxx
– – xx x x
– x – – x
Profile 4 0–20 20–42 42–75 75–121 121–146
3 1 3 1 0
21 12 15 23 11
2 0 1 2 1
6 4 4 6 3
9 17 14 12 42
17 24 43 18 0
14 14 4 14 8
27 27 15 24 35
xx xx x xx xxx
– – – x x
xx xx xx xx x
xxx xxx xxx xxx xxx
x x x – –
– – – x x
Profile 6 0–25 25–46 46–76 76–108 108–120 120–130
7 2 2 3 – 0
16 19 18 8 2 5
2 1 1 1 – 0
6 6 10 10 1 4
Nd Nd Nd Nd Nd Nd
Nd Nd Nd Nd Nd Nd
Nd Nd Nd Nd Nd Nd
Nd Nd Nd Nd Nd Nd
xx xx xx xxx xxx xxx
x x x x x x
x x xx xx xx x
xxx xxx xxx xxx xxx xxx
– – – x x x
– – – x x x
Profile 10 0–16 16–32 32–41 41–64 64–101 101–127 127–142 142–165
1 2 1 1 1 1 0 0
8 12 10 13 10 3 2 1
– 1 6 2 2 2 1 2
8 9 8 5 7 4 2 3
51 56 14 30 16 49 0 0
6 0 0 16 23 11 75 45
2 2 4 6 5 5 3 4
24 18 57 27 36 25 17 41
xxx xxx xxx xxx xxx xxx xxx xxx
xx xx xx xx xx xx xxx xxx
xx xx xx xx xxx xx xx xx
xxx xxx xxx xxx xxx xxx xxx xxx
x x x x x x x x
– – – – – x x x
Profile 14 0–30 30–52 52–93 93–110 110–128 128–172 172–179 179–204 204–209 209–218 218–200
1 1 1 1 2 4 2 1 2 0 0
4 6 5 5 9 7 7 4 3 2 1
3 2 2 1 3 2 9 3 1 1 0
4 5 6 3 9 4 17 3 2 1 1
67 45 46 75 61 82 50 83 44 92 94
– 6 9 – – – 10 – 20 – 0
3 8 4 3 1 – 2 2 6 2 1
19 25 33 13 15 3 19 4 22 2 3
Nd Nd Nd Nd Nd xxx xxx xxx xxx xxx xxx
Nd Nd Nd Nd Nd xxx x xx xx xx –
Nd Nd Nd Nd Nd xx xx xx xxx xx xxx
Nd Nd Nd Nd Nd xxx xxx xxx xxx – –
Nd Nd Nd Nd Nd x x x – x x
Nd Nd Nd Nd Nd – – – – x –
Profile 17 0–19 19–35 35–58 58–74 74–100
2 1 0 0 0
5 5 4 5 4
2 2 4 0 1
2 2 3 3 3
51 67 71 52 61
0 0 0 0 0
15 13 6 12 9
23 20 10 28 22
xxx xxx xxx xxx xxx
x x xx x xx
xx x xx xx xxx
xxx xxx xxx xxx xxx
x x – – x
– x x x x
(continued on next page)
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Table 2 (continued) Depth cm
100–125 125–140 140–158 158–214 214–240 240–250 250–300 300–420
Heavy minerals
Light minerals
Clay minerals
Op
Ol
Au
Ho
Vg
Va
Qz
Fd
Am
Cr
Fd
Ca
He
Fe
0 0 0 0 0 0 0 0
3 4 3 0 2 2 4 0
1 1 3 0 4 0 0 0
3 2 4 0 4 3 3 1
65 69 87 52 31 42 42 56
0 0 0 23 12 6 6 0
7 9 3 8 7 12 12 6
21 15 0 17 40 35 35 37
xxx xxx xxx xxx xxx xxx xxx xxx
xx x xx xx xx xx x x
xx xxx xx xx xxx xxx xxx xxx
xxx xxx xxx xxx xxx – – –
x x – – – x x x
– – x – – x x x
Op, opaque; Ol, olivine; Au, augite; Ho, hornblende; Vg, volcanic glass; Va, volcanic ash; Qz, quartz; Fd, feldspars; am, amorphous silica; Cr, cristobalite; Ca, calcite; He, hematite; Fe, ferrihydrite. Relative peak size: xxx, large; xx, medium; x, small; (–), no peaks; Nd, not determined.
(2005). For the sake of completeness; a brief description is given below. These sediments have a high moisture content on a basis of oven dry soil (314%) in Atenco, which can reach however up to 500% in the central part of the lake, a high CEC (240 cmol(+)kg 1), a high porosity (84%) and a low apparent bulk density (0.2 g/cm3) (Gutie´rrez Castorena, 1997; Gutie´rrez-Castorena et al., 2005). It has a brown colour (10YR 4/3) when moist. In thin sections (Table 4) the jaboncillo appears as light greyish limpid clay with an undifferentiated bfabric. The micromass occupies up to 90% of the groundmass, the coarse material being mainly ostracode shell fragments (Fig. 5a), forming an open porphyric c/f related distribution pattern. According to XRD the clay consists of abundance of calcite, amorphous silica, cristobalite, feldspar and iron oxides (Table 2) with sometimes traces of kaolinite. In lacustrine-alluvial layers from summit to footslope (Profiles 7, 4, 6 and 10) thin, discontinuous layers of a similar material occur, not considered as jaboncillo sensu stricto. They have the same micromorphological characteristics as jaboncillo, but the
opal and calcite (Fig. 4a and b). Some layers contain a maximum of 22.2% silica (Table 1, Profile 10, Ap horizon), occurring as yellowish brown isotropic laminae (Table 4). Micrograph of concentric of ooliths (Fig. 4c) and microprobe analyses using line scan technique (Fig. 4d) point to sharp boundary between the carbonates and the silica (Fig. 4a) indicating that successive deposits in alternating environmental conditions formed the nodules. According to XRD (Table 5) the silica of these ooliths is essentially composed of opal-CT. 3.2. Jaboncillo and thin discontinuous layers of amorphous material The lacustrine deposits, up to 70 m thick, are locally known as bjaboncilloQ (small soap) because of their soapy sensation and high water retention (Gutie´rrez Castorena, 1997). Jaboncillo occurs predominantly in the lower profiles and the deepest layers studied (1.72 m) (Profiles 14 and 17). The physical and chemical characteristics of the jaboncillo are discussed in detail by Gutie´rrez-Castorena et al.
Table 3 Percentage of the total mass of the silica features, extracted with repeated NaOH 0.5 N treatments Silica features
Silica nodules Fossilized roots Volcanic glass – not present.
Silica extracted (%) 1
2
3
4
5
6
7
8
9
10
Total
8.4 6.4 14.9
10.8 21.3 6.5
7.7 11.3 10.0
36.5 9.1 8.9
31.2 7.2 8.7
5.4 12.0 3.5
– 6.0 7.5
– 4.2 5.4
– 4.3 3.2
– 5.2 2.4
100.0 87.0 71.0
M.C. Gutie´rrez-Castorena et al. / Geoderma 132 (2006) 89–104
7
4
0
6
95
Profile 10
14
17
Depth (cm)
Symbols Ooliths
25
Ooliths with Silica Jaboncillo with mineral grains
50
Jaboncillo Duripan Infilling
75
few
Amorphous silica coatings common abundant
100
few
Silicified roots common
abundant
125
150
175
200
Fig. 3. Distribution of the opaline features in the different horizons and layers in the profiles.
groundmass contains many primary mineral grains (Fig. 5b) forming an open, double-spaced or sometimes even single spaced porphyric c/f related distribution pattern. The coarse fraction consists of volcanic glass (9 to 94%; with a average 42%), and euhedral or subhedral grains of feldspar (anorthite), olivine, hornblende and augite (Table 2). 3.3. White opaline nodules White opaline nodules are only identified in the lacustrine sediments of the central part of the lake and are always related to a jaboncillo groundmass, where they occupy up to 10% by volume. The white
nodules have a diameter of 1 to 2 cm, an irregular or lenticular shape, a soft and gelatinous touch when moist, but hard when dry, and show a banded distribution. In thin sections they are white or light brown in transmitted light and cloudy in oblique incident light (Table 4; Fig. 6a and b). SEM and EDXRA studies show that the nodules consist of an agglomeration of spheres of about 1 Am in diameter of pure opal. This composition is confirmed by XRD (Fig. 7) showing weak reflections of cristobalite and tridymite and a dominance of opal-A and by the fact that after six treatments with 0.5 N NaOH the (hard) nodules completely dissolve, releasing 100% of silica (Table 3).
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Table 4 Optical properties of opaline features Feature
Colour PPL
Limpidity PPL
Relief PPL
OIL
XPL
Ooliths Jaboncillo White nodules Roots Channel coating Fissure coating Infillings (dense complete o incomplete)
Yellow-brown Light brown White, light brown o brown Light brown, brown Light brown Light brown Yellow-brown
Limpid Speckled Limpid Limpid Limpid Limpid, speckled Limpid, speckled
negative negative negative negative negative negative negative
Cloudy Cloudy, light grey Cloudy, light grey Light grey Light grey Yellowish brown Cloudy, light grey
Iso Iso Iso Iso Iso Iso-aniso Iso-aniso
PPL: plain polarised light; OIL: oblique incident light; XPL: crossed polarised; iso: isotropic; aniso: anisotropic.
3.4. Silicified roots Silicified roots occur in the lower parts of the hill (Profiles 6 and 10) or the borders of the lake (Profile 14) and form superposed strata (Table 6) which vary in thickness from 58 to 110 cm. The silicified roots occupy 20 to 70% of the volume of the layer. Their vertical orientation (Fig. 8a) indicates that they have not been hindered in their growth and that therefore the cemented layer they traverse can only have hardened after their formation. The roots can be related to aquatic grasses because of the presence of abundant diatoms. The silicified roots are light grey (10YR 7 / 2), 0.5 mm in diameter, 2 to 7 cm long, fibrous and hard when dry but soft when moist. In thin section the silicified roots appear as series of articulated, although separated, phytholiths in their original position. All cells are prolate, isotropic and perfectly preserved, the silica completely filling their lumen (Fig. 8b). SEM studies show that in most cases it is possible to distinguish the epidermis, the parenchyma, the fibres and the vascular tissues. The poly-
hedral parenchyma cells are characteristic for grasses (Geis, 1978), although also tubular ones occur, as shown in Fig. 8c and d. Microprobe analyses show that the silicified roots are 100% silica. Similar data of 99.9% have been reported for the inorganic parts of bamboo (Jones et al., 1964). The silicified roots dissolve only partially after 10 treatments with 0.5 N NaOH, compared to six treatments for a complete dissolution of the white nodules (Table 3). The larger internal specific surface of the nodules, composed of spheres of about 1 Am may play a role in this difference. These findings also support the idea of Drees et al. (1989) that biogenic and phytogenic opal is less soluble than inorganic opal. According to XRD (Table 5; Fig. 9) only traces of tridymite and cristobalite are present, pointing to an opal-A. In the roots under study all cellulose and lignin has been replaced by silica. The process was rather rapid, as the sediments are less than 1000 years old, based in the presence of underlying buried prehispanic constructions.
Table 5 Mineralogical and chemical composition of opaline features by X-ray diffraction and EDXRA Feature
Amorphous
Cristobalite
Tridymite
Others
EDXRA
Ooliths Jaboncillo White nodules Silicified roots Channel coating Fissure coat Infillings
x xxx xxx xxx xx xx xx
x xx x x xx xx xx
x – x x – – –
Cal. (xxx) Fd., Go., Lp., He., Cal., Do. – – – – –
Ca, Si, Al, Mg Si, Al, Mg, Na, K, Fe, Cu Si Si Si, Ca Si, Ca Si, Ca
Go: goethite; Lp: lepidocrocite; He: hematite; Cal: calcite; Do: dolomite; Fd: feldspars. Relative peak size: xxx, large; xx, medium; x, small; (–) no peaks.
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Fig. 4. Micrograph of concentric calcitic opaline ooliths in thin section, (o) opal, (cc) calcium carbonate; profile 6, 2CBk. a) PPL, b) XPL, frame length 2.9 mm; c) BSI of oolith in thin section with position of line scan; d) line scan for Si, Ca and Al.
3.5. Opaline coatings related to channels Microlaminated isotropic coatings occur around the channels related to the silicified roots (Fig. 8a and b). They are limpid (Table 4) light brown coatings, which have sharp boundaries with the groundmass. The microlaminations are oriented parallel to
the channel wall, indicating a sequential deposition with approximately regular thickness; they are common (Profiles 4 and 6) to abundant (Profile 10) with a random basic distribution. According to XRD (Table 5) cristobalite and opal-A are the main constituents. The channels are surrounded by diffuse iron and/or manganese hydroxide hypocoatings.
Fig. 5. a) Micrograph of jaboncillo: isotropic groundmass and fragment of ostracods in XPL, frame length 5.3 mm, Profile 14, 6Ck; b) dense complete infilling with opal, XPL, frame length 5.3 mm, Profile 14, 2CBk.
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Fig. 6. Opal nodules in jaboncillo of the central part of the lake. a) Lenticular nodule in oblique incident light; b) other nodule in XLP. Frame length 2.9 mm.
cm high) where opal and clay infilling form mounds, up to 10 cm high. These materials are not indurated, but soft with a slippery consistency when moist, and can easily be separated. Micromorphological studies show that these (2.5Y 7 / 6) macro-infillings consist of yellowish brown, isotropic or weakly anisotropic, 300 to 500 Am thick microlaminated coatings (Table 4), indicating a sequential deposition (Fig. 10a, b and c). Most dominant are the isotropic coatings, and those
3.6. Opaline coatings related to fissures In the lower parts of the profiles on the shoulder and foot slope (Profile 4 and 6), opal forms coatings, up to 2 mm thick and 30 cm long. They are mainly related to horizontal fissures between layers and vertical fissures inside a petrocalcic horizon (Profile 10, from 2CBkm to 5Cqk). In the deepest part (160 cm) of Profile 10 a fissure is connected to a crotovina (25 cm wide and 20
4.3
3.8
a 4.1* 4.04** 3.8* 4.3*
3.1**
5
10
15
20
25
30
b
2.4**
2.9*
35
40
Degrees 2 Fig. 7. XRD diffraction. a) silica nodule, b) fossilized root.
45 * Tridymite ** Cristobalite
50
M.C. Gutie´rrez-Castorena et al. / Geoderma 132 (2006) 89–104 Table 6 Presence of silicified roots layers and their thickness in the studied profiles Profile number
Number of root layers
Cumulated thickness of root layers in cm
7 4 6 10 14 17
– – 5 5 4 3
– – 105 110 99 58
showing randomly oriented, weakly anisotropic domains. Less commonly the anisotropism occurs as continuous streaks parallel to the walls, and with a length slow orientation. Between crossed polarizers and with the help of a retardation plate, a moderate cross-juxtaposition can be noticed. In oblique incident light these coatings can be distinguished from those around channels by their yellowish brown colour, compared to the pale yellow colour in the channel coatings, and the presence of anisotropic domains and streaks. The opaline coatings are a mixture of opal-A and cristobalite (Table 5). 3.7. Microlaminated infillings between minerals and ooliths Laminated infillings occur in the soils on the lower parts of the hill (Profiles 6 and 10). They are related morphologically to the coatings and infillings of fissures, as observed in the field and described above, but on a much smaller scale, and only visible at close observation. The opal-A and cristobalite (Table 5) forms microlaminated coatings and infillings in fissures (macro-features), interstices between calcite nodules (described earlier in this paper) and minerals of the sandy layers in profiles 6 and 10, creating a close porphyric c/f related distribution pattern. They are 1 to 2 mm thick and composed of 50 to 150 Am thick microlaminations, comparable to those described for fissures (Figs. 11 and 12). 3.8. Other siliceous components Volcanic glass (Table 2) is part of the sedimentary units (alluvial and lacustrine). It is the principal
99
constituent of the rocks in the surrounding mountains (Rodrı´guez et al., 1999) and was transported by rivers or wind. Residues of diatoms and phytoliths are also abundant, especially in the footslope (Profiles 14 and 17). The phytoliths have polyhedral forms, as common for grasses (Geis, 1978) and they are similar to those observed in the fossilised roots. Although phytoliths of a forest environment have been reported by Rodrı´guez (1999) in the mountainous zone, they were probably not transported to the lake.
4. Discussion The sediments of the former Texcoco Lake, and the soils formed on them are rich in siliceous features of different morphologies and different origins. Some were formed in the highly alkaline former Texcoco Lake previous to its drainage (Del Valle, 1983), other after emergence. The former lake was situated in a closed basin, surrounded by rocks with a high content of acid volcanic glass (70%), which upon weathering released amorphous silica (Rodrı´guez et al., 1999). Amorphous silica concentration in the soils increase from 6% in the surrounding mountains (Segura et al., 2000) up to 45% in the lake (Del Valle, 1983). The global geochemical landscape is characterised by an accumulation of Al in the higher, moister forest soils (e.g. as kaolinite), a retention of Mg in the smectitic soils on the lower, dryer slopes (Segura et al., 2000), and an accumulation of Si, Ca and Na in the basin lake (Del Valle, 1983). 4.1. Ooliths Because of their stratigraphic position these calcitic-opaline features with amorphous silica can be related to the beaches of the former lake (Gutie´rez Castorena et al., 1998). They most probably formed during the last 1000 years as can be deduced from the presence of artefacts, which also proves their sedimentary origin. The area was characterised by an intense evaporation (Sanders, 1983) and a high biological activity (presence of silicified aquatic grasses). In lacustrine environments the evaporation and photosynthesis of aquatic plants, which removes CO2, promotes the precipitation of CaCO3 (Bathurst,
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Fig. 8. Silicified roots (Profile 14, 4Ck). a) Soil profile; b) thin section of transversal section through silicified root. PPL, frame length 5.3 mm; c) and d) SEM images showing cells of parenchyma (p), fibers (f) and xylem (x).
1975). Concentric calcite nodules, called ooliths, are formed in shallow agitated water, and are generally associated with micro-organisms (Drees and Wilding, 1987). A decrease of the pH, provoked by the input of pluvial water in the alkaline lake, may be responsible for the precipitation of Si (Siever, 1957; Jones et al., 1967). The repeated input of fresh water, followed by evaporation and biological activity can explain the alternation of sharp bounded calcitic and siliceous laminae. A diagenetic transformation of opal-A may
have been taken place by the presence of cristobalite and tridymite (Table 5). 4.2. Jaboncillo According to Gutie´rrez-Castorena et al. (2005) the jaboncillo is composed of amorphous material of colloidal size that precipitated during the major extension of the lake as a result of small environmental changes (slight evaporation and moderate pH changes
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101
Fig. 9. Coating of microlaminated opal in channel. a) PPL; b) XPL. Frame length 2.9 mm.
to less alkaline conditions). The thin discontinuous lacustrine alluvial layers at the footslope, having the same characteristics as the jaboncillo, were most probably formed under similar conditions. It is clearly a mixture of opal-A with amorphous iron (hydr)oxides and mineral particles of colloidal size. The lower water retention capacity of the jaboncillo in Atenco (at the border of the lake) compared to that in the centre of the lake, points to a relative irreversible dehydration of the sediments at the lake’s edge.
4.3. Nodules White nodules occur in the original sediment as soft, whitish concentrations of silica, pointing to an early diagenetic process of Si accumulation. There is no morphological indication in the surrounding sediment that they could be synsedimentary. They seem to be composed essentially of opal-A. The hardening may be explained by a loss of water resulting in a collapse of the randomly packed opaline spheres giv-
Fig. 10. a) Fragment of coating of opal (o) with fissures infilled by calcite (cc), PPL, frame length 2.9 mm; b) same, but XPL; c) microlaminated coating of clay and opal infilling of crotovina, PPL, frame length 5.3 mm. Profile 10, 5Cqk.
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Fig. 11. a) Dense complete infilling of opal between ooliths. PPL, frame length 2.9 mm; b) same but XPL. Profile 6, Bkqm.
ing rise to closer packing. As they harden upon drying, the presence of soft nodules in the field proves that the layers were never exposed. 4.4. Silicified roots Silicified roots were formed at several intervals, partly in historic times in the lake, pointing to a frequent change in shoreline position. It is an indication for the stability of biogenic opal-A that they are not yet transforming to opal-CT, although younger features, such as pedogenic coatings, are already in that stage of evolution. The fact that numerous similar phytoliths are found in the surrounding material indicates that silicification took place at least partially in the living plant. It has been reported that plants impregnate their epidermis with silica in order to reduce losses of water by transpiration (Raven, 1983) and/or to reduce damage by salinity (Ahmad et al., 1992). Silica is absorbed by
a
the plants as H4SiO4 (Alexander et al., 1954) and transported through the xylem. It can however not be excluded that after burial more silica precipitated from the environment. The formation of microlaminated coatings in the root channels prove that silicification took place before the period of high silica mobility after emergence of the sediments. 4.5. Opaline coatings on channels and fissures The distribution of coatings and infillings is clearly related to pedogenic porosity (shrinkage cracks, biopores), which can only be formed after emergence of the sediments. As the surrounding material seems unweathered, as evidenced by the presence of high quantities of volcanic glass and euhedral olivine (Table 2) the origin of the amorphous silica must be related to the lacustrine jaboncillo layers. An alkalisation of the sediment near the surface after the evaporation of the lake may have caused a solubilisation of
b
Fig. 12. a) Dense complete opal of packing voids, PPL, frame length 5.3 mm; b) same, but XPL. Profile 10, 2CBkqm.
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the silica which, after transport to deeper parts of the soil profiles has precipitated in the zones with a lower pH, and moreover started to crystallise as cristobalite, explaining partially the weak anisotropy (Flach et al., 1969). At the summit position (Profile 7) a strong alkaline pH is measured in which silica is more soluble, and low SiO2/Al2O3 molar ratios (from 5.8 to 24.2), whereas at the base the pH is only slightly alkaline and the SiO2/Al2O3 molar ratios are much higher (up to 199.2) (Profile 10) (Table 1). A transport of Si from the upper part of the lower profile might explain the opal infillings and coatings of the horizontal fissures and the intense accumulation in the lower part, including the infilling of the crotovina. The coatings resemble part of the bmilky pedofeaturesQ described by Usai and Dalrymply (2003). The weak anisotropism can be related to an initial crystallisation of the opal-A to opal-CT (anisotropic domains), or to the presence of fine clay particles oriented according to the flow direction of the colloids (parallel anisotropic streaks). The cross-juxtaposition of the laminae, and the alternation of limpid isotropic and less limpid, weakly anisotropic laminae, points to a complicated, sequential illuviation of silica, maybe partially as solution, partially as colloid. 4.6. Opal infillings In the oolithic layers two generations of opal are present: in the nodules, where opal and calcite accumulated alternatively in the same feature as a result of environmental changes in the lake, and as microlaminated coatings formed by sequential deposition of silica between the coarse elements by pedogenic processes, as described above. The silica precipitates first around the ooliths and mineral grains and later these coatings serve as a base for further deposition creating locally a close porphyric c/f related distribution pattern as mentioned by Chadwick et al. (1987), who describe infillings of up to 150 Am thick in soils with a sandy texture. In the studied soils however the infillings are much larger and related to macropores such as fissures and packing voids. The siliceous coatings and infillings form a horizon with the properties of a duripan (Soil Survey Staff, 1999). It is indurated and slakes in concentrated KOH. These horizons normally form in xeric and arid
103
moisture regimes or in volcanic soils where weathering processes dissolve volcanic glass and liberate soluble silica which precipitate at a given depth. In this case the bduripanQ is formed in an ustic moisture regime and the soluble silica originates from lacustrine sediments rich in this element by lateral transport. The alkaline pH provoked the resolubilisation and the natural relief the redistribution along the slope. As the roots of aquatic plants traverse vertically these cemented layers, it is evident that the duripan postdates the lacustrine environment and therefore must be probably less than 100 years old.
5. Conclusions The Ex-Lago de Texcoco was situated in a closed basin composed of igneous rocks with a high amount of acid volcanic glass (up to 70%), which upon weathering released silica that was transported to the lake and became saturated with this element. In the deepest parts of the lake the silica was precipitated in colloidal form due to small changes in the environment (pH and evaporation) giving rise to an optically isotropic material called bjaboncilloQ. This material is also found in the deepest horizons of the studied soils, closer to the shore, decreasing in thickness towards the summit. Diagenetic concentrations of pure silica formed in the jaboncillo as soft nodules of opal-A, hardening irreversibly upon drying. In old beach ridges, changes in pH, in evaporation and in biological activity lead to sequential deposition of calcium carbonate and opal, forming concentric ooliths. Aquatic grasses whose roots penetrated the lacustrine sediments rich in amorphous silica invaded the borders. The plants absorbed significant amounts of Si and were silicified as opal-A. This process took place during different periods as different superposed strata were formed, partly younger than some prehispanic constructions. Once the sediments emerged, either by natural or by artificial processes, they gave rise to materials with a strong alkaline pH, allowing the Si to go into solution and to migrate to deeper horizons or to move laterally, concentrating in the soils of the lower part of the hill slope, where polygenetic opaline coatings and infillings were formed in pedogenic pore spaces, locally forming a duripan.
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References Ahmad, R., Zaer, H.S., Shoaib, I., 1992. Role of silicon in salt tolerance of wheat (Triticum aestivum L). Plant Science 85, 43 – 50. Alexander, G.B., Heston, W.M., Iler, R.K., 1954. The solubility of amorphous silica in water. Journal of Physical Chemistry 58, 453 – 455. Bathurst, R.G.C., 1975. Carbonate sediments and their diagenesis, Second ed. Developments in Sedimentology, vol. 12. Elsevier, Amsterdam, The Netherlands. Bullock, P., Federoff, N., Jongerius, A., Stoops, G., Tursina, T., Babel, U., 1985. Handbook for Soil Thin Section Description. Waine Research Publications, Wolverthamton, England. Chadwick, O.A., Hendricks, D.M., Nettleton, W.D., 1987. Silica in duric soils: I. A. Depositional model. Soil Science Society of America Journal 51, 975 – 982. Del Valle, C.H. 1983. Los procesos de acumulacio´n de sales e intemperismo en cubetas lacustres, en la zona de transicio´n del exlago de Texcoco. MSc Thesis. Colegio de Postgraduados. Chapingo, Me´xico. Departamento del Distrito Federal, 1967–1975. Memorias Del Sistema De Drenaje Profundo Del D.F. Secretarı´A De Obras y Servicios. Direccio´n General de Obras Hidra´ulicas, Me´xico, D.F. Drees, L.R., Wilding, L.P., 1987. Micromorphic record and interpretations of carbonate forms in the Rolling Plains of Texas. Geoderma 40, 157 – 175. Drees, L.R., Wilding, L.P., Smeck, N.E., Senkayi, A.L., 1989. Silica in soil: quartz and disordered silica polymorphs. In: Dixon, J.B., Weed, S.B. (Eds.), Minerals in Soil Environments, Soil Science Society of America Book Series, vol. 1. SSSA, Madison, WI, pp. 913 – 974. Flach, K.W., Nettleton, W.D., Gile, L.H., Cady, J.G., 1969. Pedocementation: induration by silica, carbonates and sesquioxides in the Quaternary. Soil Science 107, 442 – 453. Garcia, E., 1968. Los climas del Valle de Me´xico Segu´n el Sistema de Clasificacio´n Clima´tica de Kopen Modificado. Colegio de Postgraduados, Chapingo, Me´xico. Geis, J.W., 1978. Biogenic opal in three species of Gramineae. Annals of Botany 42, 1119 – 1129. Gutie´rrez Castorena, Ma del C., 1997. Los suelos de la ribera del ex lago de Texcoco (Macro y micromorfologı´a). Ph.D. Thesis. Programa de Edafologı´a, Colegio de Postgraduados. Montecillo. Texcoco, Me´xico.
Gutie´rez Castorena, Ma del C., Stoops, G., Ortiz Solorio, C.A., 1998. Carbonato de calcio en los suelos del ex lago de Texcoco. Terra 16, 11 – 19. Gutie´rez-Castorena, Ma del C., Stoops, G., Ortiz Solorio, C.A., Lo´pez, A.G., 2005. Amorphous silica materials in soils and sediments of the Ex-lago de Texcoco, Mexico. Catena 60, 205 – 226. Jackson, M.L., 1965. Free oxides, hydroxides, and amorphous alumino-silicates. In: Black, C.A. (Ed.), Methods of Soil Analysis. Part. 1, Agronomy, vol. 9, pp. 578 – 603. Jones, J.B., Sanders, J.V., Segnit, E.R., 1964. Structure of opal. Nature 204, 990 – 991. Jones, F.B., Retting, S.L., Eugste, H.P., 1967. Silica in alkaline brines. Science 158, 1310 – 1314. Mooser, F., 1963. Informe sobre la geologı´a de la cuenca del Valle de Me´xico. D.F.. Secretarı´a de Recursos Hidra´ulicos, Comisio´n Hidrolo´gica de la cuenca del Valle de Me´xico, Me´xico, D.F. Murphy, C.P., 1986. Thin Section Preparation of Soil and Sediments. AB Academic Publishers, Berkhamsted, Great Britain. Raven, D.A., 1983. The transport and function of silicon in plants. Biological Reviews of the Cambridge Philosophical Society 58, 179 – 207. Rodrı´guez, T.S. 1999. Proceso de intemperismo en tepetates y su influencia en la formacio´n de suelos. MSc. Thesis. Programa de Edafologı´a. Colegio de Postgraduados, Montecillo, Me´xico. Rodrı´guez, T.S., Gutie´rrez-Casterona, Ma. del C., Hidalgo, M.C., Ortiz Solorio, C.A., 1999. Intemperismo en tepetates y en cenizas volca´nicas y su influencia en la formacio´n de andisoles. Terra 17, 97 – 108. Sanders, H.J., 1983. Reorganizing and improving environmental condition in the high Valley of Mexico. Applied Geography and Development 16, 105 – 116. Segura, C.M.A., Gutie´rrez-Castorena, Ma. del C., Ortiz-Solorio, C.A., Go´mez-Diaz, D.J., 2000. Suelos Arcillosos de la Zona Oriente del Estado de Me´xico. Terra 18, 35 – 44. Siever, R., 1957. The silica budget in the sedimentary cycle. American Mineralogist 42, 821 – 841. Soil Survey Staff, 1999. Soil Taxonomy, 2nd edition. USDA. Natural Resources Conservation Service. Stoops, G., 2003. Guidelines for the Analysis and Description of Soil and Regolith Thin Sections. SSSA, Madison, WI. Usai, M.R., Dalrymply, J.B., 2003. Characteristics of silica-rich pedofeatures in a buried paleosol. Catena 54, 557 – 571. Van Reeuwijk, L.P., 1995. Procedures for Soil Analysis, 4th ed. Tech. Pap., vol. 9. ISRIC, Wageningen, The Netherlands.
Micromorphology of opaline features in soils on the sediments of the ex-Lago de Texcoco, Me´xico Ma del Carmen Gutie´rrez-Castorenaa,*, G. Stoopsb, C.A. Ortiz-Solorioa, P. Sa´nchez-Guzma´na a
Soil Science Programme, Colegio de Postgraduados, Montecillo, Texcoco 56230, Me´xico Laboratorium voor Mineralogie, Petrologie en Micropedologie, Ghent University, Krijgslaan 281, S8, 9000 Gent, Belgium
b
Received 5 November 2003; received in revised form 29 April 2005; accepted 4 May 2005 Available online 14 June 2005
Abstract The former Texcoco Lake (ex-Lago de Texcoco) is situated in the closed Basin in Mexico. The lake was artificially drained in the 20th century. Six soil profiles on the eastern border of the former lake were analysed for mineralogical and microfabric characteristics in order to differentiate between sedimentary and pedogenic processes. Large quantities of silica liberated as a result of weathering of the surrounding volcanic mountains partly precipitated in the lake. As a result, layers of an exceptional sediment, locally called bjaboncilloQ, were deposited. They are characterised by a high Si-content and a very high (up to 500%) water retention. In addition several amorphous silica features were formed in these lacustrine conditions. In the centre of the lake opaline nodules, mainly composed of opal-A, formed in the jaboncillo. Near the former coast line concentric silico-calcitic nodules (ooliths) were formed as a result of fluctuations of the input of fresh water and biological activity. Several superposed layers of in situ silicified roots point to frequent changes in the shore line position. After emergence of the sediments, about 100 years ago, a translocation of silica, mainly caused by an alkalisation of the sediment, gave rise to the formation of different pedogenic types of opaline coatings and infillings, resulting locally in a duripan. The micromorphological and mineralogical properties of these silica features are discussed and sedimentary and pedogenic processes considered. D 2005 Elsevier B.V. All rights reserved. Keywords: Jaboncillo; Nodules; Ooliths; Silicified roots
1. Introduction * Corresponding author. Programa de Edafologı´a, Colegio de Postgraduados Montecillo, Texcoco 56230, Me´xico. Tel.: +52 595 95 1 14 74; fax: +52 595 95 1 01 98. E-mail address: [email protected] (M.C. Gutie´rrez-Castorena). 0016-7061/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2005.05.002
Mexico City was built in the Texcoco Lake and since prehispanic times subject to frequent flooding. In the beginning of the 20th century the lake was artifi-
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cially drained. In the strongly alkaline sediments of the former Texcoco Lake the presence of amorphous siliceous materials, mainly of inorganic origin, has been reported, yet not described or discussed (Del Valle, 1983; Gutie´rrez Castorena, 1997; Segura et al., 2000; Gutie´rrez-Castorena et al., 2005). One of these materials is a buried lacustrine sediment layer, locally called bjaboncilloQ, up to 70 m thick and extending over an area of approximately 400 km2 (Departamento del Distrito Federal, 1967–1975). As these materials emerged following drainage about 100 years ago, the sedimentary processes leave have an important imprint on the profile morphology, and pedogenic processes are only in an incipient phase (Gutie´rrez Castorena, 1997). Siliceous features occurring in these sediments, or related to them and formed by sedimentary, biological or pedogenic processes, have not yet been discussed in literature. The objective of the present study is to investigate the relationship of the opaline features with their past and present position in the landscape of the former Texcoco Lake, to distinguish between sedimentary and pedogenic features and to determine the processes involved in their formation.
2. Materials and methods 2.1. Location The study was carried out in the community of Atenco, a site considered as part of the border of the ancient Texcoco Lake in prehispanic times. It is situated in the Mexico Basin, at 2,289 m above sea level (Fig. 1). The area consists of Quaternary lacustrine and alluvial sediments of volcanic origin (Mooser, 1963) and is surrounded by mountains composed of volcanic rocks such as vitreous tuffs (tepetates), rhyolites and andesites. The climate is semi-arid BS KW (W)(i) (Garcı´a, 1968) with a mean annual precipitation of 600 mm and a mean annual temperature of 18 8C. The soils are classified as Petrocalcic Calciustolls (Gutie´rrez Castorena, 1997) and are mainly used for dry land farming. 2.2. Fieldwork Based on a survey, six soil profiles were selected: four on a 400 m long sequence on the Tepetzingo hill, and two on the shore of the ancient lake, close to the village of Atenco (Fig. 2). The Tepetzingo hill, which 1
0
2
3 Km
19°35’
N
Tepetzingo hill
Atenco Village
Huatepec hill
Texcoco Village Ex lago de Texcoco
Mexico State 98°57’ Mexico City
19°30’ 98°53’ Sequence line of soil profiles Border of ex lago de Texcoco during prehispanic times
Fig. 1. Location of the study area.
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91
Tepetzingo Hill
7 20 m
4 6
10
Alluvial and lacustrine sediments Basaltic rock
14 Lacustrine sediments
17
Atenco Village
Prehispanic ruins
400 m
3 km
Fig. 2. Location of the profiles on Tepetzingo hill (left) and near Atenco village (right).
now rises 10 m above the plain, is an ancient basaltic volcanic island partially covered by alluvial and lacustrine materials. It bears a relict cactaceous vegetation. Each soil horizon or layer was sampled for physical and chemical analyses and oriented, undisturbed samples were obtained for micromorphological studies.
studied by X-ray diffraction after the oxidation of organic matter. Carbonates and iron oxides were analysed in the samples and, they were removed with acetate buffer solution 1 M, dithionite and acid oxalate prior to the phyllosilicate clay analysis (Van Reeuwijk, 1995). No saturations with cations or glycol were made.
2.3. Analyses 3. Results Grain size distribution was determined by the pipette method, pH water measured at a soil/water ratio of 2 / 1, and CaCO3 by acid neutralisation (Van Reeuwijk, 1995). Silica and aluminum in noncrystalline or poorly crystalline forms were extracted by boiling 500 mg of the b 2 mm fraction in 0.5 N NaOH for 2.5 min (Jackson, 1965). Silica and aluminum were measured by Atomic Absorption Spectroscopy (AAS). The undisturbed samples were air dried prior to impregnation with polyester resin and monostyrene (7 : 1 ratio) and thin sections prepared according to the methods of Murphy (1986). Concepts and terminology of Bullock et al. (1985) and Stoops (2003) were applied for the micromorphological study. Special features were also studied with a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analysis system (EDXRA) and by X-ray diffraction (XRD). The sand fraction (250–500 Am) was separated with bromoform (specific density = 2.89 kg/dm3) into a light and a heavy fraction. On grain mounts, 200 grains were counted for each fraction according to the line counting methods. The clay fraction was
A great variety of opaline features were observed, partly of sedimentary, partly of pedogenic origin and varying according to their position in the landscape. Some of the features can be observed readily in the field, at a macroscale whereas other require a more microscopic observation. The physical, chemical and mineralogical analyses of the studied soils are presented in Tables 1, 2 and 3. The distribution of the opaline features is shown in Fig. 3, and their optical and mineralogical properties in Tables 4 and 5. 3.1. Calcitic-opaline ooliths Oolithic (Profiles 4, 6, 7 and 10) layers, 40 to 150 cm thick, were observed from summit (Profile 7) to the footslope (Profile 10) of the hill (Fig. 3). Ooliths occupy from 5 to 80% (volume) of these layers and are the main constituent of the sand fraction, besides euhedral and subhedral grains of plagioclase, volcanic glass, olivine, hornblende and augite (Table 2). The ooliths are about 1 mm in diameter and appear in thin sections as concentric calcitic nodules or nodules with an alternation of
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Table 1 Selected chemical and physical data Horizon
Depth cm
Grain size distribution %
pH 2 : 1
CaCO3%
SiO2%
Al2O3%
SiO2/Al2O3
Sand
Silt
Clay
0–23 23–44 44–58 58–87 87–130
44.4 40.0 40.2 39.9 23.9
47.2 48.3 49.7 45.2 61.5
8.4 11.7 10.1 14.9 14.6
8.2 8.6 9.4 9.7 9.8
18.1 21.9 17.3 20.7 13.8
13.1 10.0 8.2 10.7 5.5
1.6 0.7 0.7 1.2 1.6
13.8 24.2 18.7 15.1 5.8
Profile 4 Ap 2Ckm 3Ckm 4Ckm 5Ck
0–20 20–42 42–75 75–121 121–148
44.7 39.9 48.6 47.3 62.5
40.7 44.6 35.0 35.3 22.8
14.6 15.5 16.4 17.4 14.7
8.0 9.3 9.2 9.1 8.2
33.3 17.1 29.5 21.0 36.8
13.2 9.0 10.9 15.3 14.2
1.3 0.6 0.6 0.6 0.5
17.0 27.2 30.3 46.2 48.4
Profile 6 Ap Bkqm 2CBk 3CBk 3CBk2
0–25 25–40 40–62 62–102 102–139
42.0 45.6 80.0 35.7 59.7
49.0 40.3 11.5 36.2 27.1
9.0 14.1 8.5 28.1 13.2
8.6 8.6 9.1 9.2 9.3
23.4 43.0 16.2 29.0 34.8
8.9 18.1 17.7 8.9 7.9
0.4 0.7 0.9 0.9 0.9
38.0 44.3 34.6 16.6 14.4
Profile 10 Ap 2CBkm 2CBkqm 3CBk 4CBk 5CBk 5Cqk2 6Ckm
0–16 16–32 32–41 41–64 64–101 101–127 127–142 142–165
50.9 81.9 81.0 91.5 81.9 60.4 61.7 70.1
20.5 13.4 12.8 4.6 12.7 25.2 20.0 16.4
28.6 4.7 6.2 3.9 5.4 14.4 18.3 13.5
7.8 8.0 8.1 8.1 8.0 7.5 7.4 7.4
19.9 23.1 29.6 30.2 31.8 17.2 22.7 22.9
22.2 26.3 15.1 6.3 12.8 Nd 25.5 7.9
2.0 0.2 0.7 0.3 0.4 Nd 0.6 0.6
19.1 199.2 38.8 38.7 50.9 Nd 70.6 23.1
Profile 14 Ap A2 AB 2CBk 3Ck
0–30 30–52 52–93 93–110 110–128
34.1 24.7 20.5 40.0 21.0
47.3 54.5 54.9 46.3 63.4
18.6 20.8 24.6 13.7 15.6
7.9 8.2 7.9 7.5 7.6
16.6 11.2 16.6 28.3 35.5
3.9 7.5 6.9 9.8 17.9
0.3 0.3 0.4 0.9 0.9
19.4 37.8 27.1 18.3 34.2
Profile 17 Ap 2A 3CBkqm 3CBkqm2 4Ck 5Ckm 6Ckm 7Ckm
0–19 19–35 35–58 58–74 74–100 100–125 125–140 140–158
59.6 48.0 67.5 68.6 74.3 80.8 74.2 33.9
31.1 38.7 25.9 27.7 21.8 14.9 20.7 43.0
9.3 13.3 6.6 3.7 3.9 4.3 5.1 23.1
8.5 8.4 8.5 8.5 8.4 8.4 8.4 8.0
16.9 19.1 38.4 32.3 32.4 33.6 36.8 37.4
7.3 8.7 10.8 18.9 12.7 14.4 21.6 21.5
1.4 1.3 1.0 0.6 0.5 0.3 1.3 1.2
8.5 11.0 30.7 54.1 40.6 72.5 29.3 30.1
Profile 7 A 2Ck 3Ck 4Ck 5CkR
Nd: not determined.
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93
Table 2 Mineralogical analysis of sand (250–500 Am) and clay fraction (b2 Am) Depth cm
Heavy minerals
Light minerals
Clay minerals
Op
Ol
Au
Ho
Vg
Va
Qz
Fd
Am
Cr
Fd
Ca
He
Fe
Profile 7 0–23 23–44 44–58 58–87 87–130
1 2 1 1 0
9 17 7 8 7
2 1 2 2 3
5 11 5 5 7
39 23 53 43 25
21 26 8 17 15
5 5 5 7 3
17 24 19 16 39
x x x x xx
– – – x xx
xx xx x x xx
xxx xxx xxx xxx xxx
– – xx x x
– x – – x
Profile 4 0–20 20–42 42–75 75–121 121–146
3 1 3 1 0
21 12 15 23 11
2 0 1 2 1
6 4 4 6 3
9 17 14 12 42
17 24 43 18 0
14 14 4 14 8
27 27 15 24 35
xx xx x xx xxx
– – – x x
xx xx xx xx x
xxx xxx xxx xxx xxx
x x x – –
– – – x x
Profile 6 0–25 25–46 46–76 76–108 108–120 120–130
7 2 2 3 – 0
16 19 18 8 2 5
2 1 1 1 – 0
6 6 10 10 1 4
Nd Nd Nd Nd Nd Nd
Nd Nd Nd Nd Nd Nd
Nd Nd Nd Nd Nd Nd
Nd Nd Nd Nd Nd Nd
xx xx xx xxx xxx xxx
x x x x x x
x x xx xx xx x
xxx xxx xxx xxx xxx xxx
– – – x x x
– – – x x x
Profile 10 0–16 16–32 32–41 41–64 64–101 101–127 127–142 142–165
1 2 1 1 1 1 0 0
8 12 10 13 10 3 2 1
– 1 6 2 2 2 1 2
8 9 8 5 7 4 2 3
51 56 14 30 16 49 0 0
6 0 0 16 23 11 75 45
2 2 4 6 5 5 3 4
24 18 57 27 36 25 17 41
xxx xxx xxx xxx xxx xxx xxx xxx
xx xx xx xx xx xx xxx xxx
xx xx xx xx xxx xx xx xx
xxx xxx xxx xxx xxx xxx xxx xxx
x x x x x x x x
– – – – – x x x
Profile 14 0–30 30–52 52–93 93–110 110–128 128–172 172–179 179–204 204–209 209–218 218–200
1 1 1 1 2 4 2 1 2 0 0
4 6 5 5 9 7 7 4 3 2 1
3 2 2 1 3 2 9 3 1 1 0
4 5 6 3 9 4 17 3 2 1 1
67 45 46 75 61 82 50 83 44 92 94
– 6 9 – – – 10 – 20 – 0
3 8 4 3 1 – 2 2 6 2 1
19 25 33 13 15 3 19 4 22 2 3
Nd Nd Nd Nd Nd xxx xxx xxx xxx xxx xxx
Nd Nd Nd Nd Nd xxx x xx xx xx –
Nd Nd Nd Nd Nd xx xx xx xxx xx xxx
Nd Nd Nd Nd Nd xxx xxx xxx xxx – –
Nd Nd Nd Nd Nd x x x – x x
Nd Nd Nd Nd Nd – – – – x –
Profile 17 0–19 19–35 35–58 58–74 74–100
2 1 0 0 0
5 5 4 5 4
2 2 4 0 1
2 2 3 3 3
51 67 71 52 61
0 0 0 0 0
15 13 6 12 9
23 20 10 28 22
xxx xxx xxx xxx xxx
x x xx x xx
xx x xx xx xxx
xxx xxx xxx xxx xxx
x x – – x
– x x x x
(continued on next page)
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Table 2 (continued) Depth cm
100–125 125–140 140–158 158–214 214–240 240–250 250–300 300–420
Heavy minerals
Light minerals
Clay minerals
Op
Ol
Au
Ho
Vg
Va
Qz
Fd
Am
Cr
Fd
Ca
He
Fe
0 0 0 0 0 0 0 0
3 4 3 0 2 2 4 0
1 1 3 0 4 0 0 0
3 2 4 0 4 3 3 1
65 69 87 52 31 42 42 56
0 0 0 23 12 6 6 0
7 9 3 8 7 12 12 6
21 15 0 17 40 35 35 37
xxx xxx xxx xxx xxx xxx xxx xxx
xx x xx xx xx xx x x
xx xxx xx xx xxx xxx xxx xxx
xxx xxx xxx xxx xxx – – –
x x – – – x x x
– – x – – x x x
Op, opaque; Ol, olivine; Au, augite; Ho, hornblende; Vg, volcanic glass; Va, volcanic ash; Qz, quartz; Fd, feldspars; am, amorphous silica; Cr, cristobalite; Ca, calcite; He, hematite; Fe, ferrihydrite. Relative peak size: xxx, large; xx, medium; x, small; (–), no peaks; Nd, not determined.
(2005). For the sake of completeness; a brief description is given below. These sediments have a high moisture content on a basis of oven dry soil (314%) in Atenco, which can reach however up to 500% in the central part of the lake, a high CEC (240 cmol(+)kg 1), a high porosity (84%) and a low apparent bulk density (0.2 g/cm3) (Gutie´rrez Castorena, 1997; Gutie´rrez-Castorena et al., 2005). It has a brown colour (10YR 4/3) when moist. In thin sections (Table 4) the jaboncillo appears as light greyish limpid clay with an undifferentiated bfabric. The micromass occupies up to 90% of the groundmass, the coarse material being mainly ostracode shell fragments (Fig. 5a), forming an open porphyric c/f related distribution pattern. According to XRD the clay consists of abundance of calcite, amorphous silica, cristobalite, feldspar and iron oxides (Table 2) with sometimes traces of kaolinite. In lacustrine-alluvial layers from summit to footslope (Profiles 7, 4, 6 and 10) thin, discontinuous layers of a similar material occur, not considered as jaboncillo sensu stricto. They have the same micromorphological characteristics as jaboncillo, but the
opal and calcite (Fig. 4a and b). Some layers contain a maximum of 22.2% silica (Table 1, Profile 10, Ap horizon), occurring as yellowish brown isotropic laminae (Table 4). Micrograph of concentric of ooliths (Fig. 4c) and microprobe analyses using line scan technique (Fig. 4d) point to sharp boundary between the carbonates and the silica (Fig. 4a) indicating that successive deposits in alternating environmental conditions formed the nodules. According to XRD (Table 5) the silica of these ooliths is essentially composed of opal-CT. 3.2. Jaboncillo and thin discontinuous layers of amorphous material The lacustrine deposits, up to 70 m thick, are locally known as bjaboncilloQ (small soap) because of their soapy sensation and high water retention (Gutie´rrez Castorena, 1997). Jaboncillo occurs predominantly in the lower profiles and the deepest layers studied (1.72 m) (Profiles 14 and 17). The physical and chemical characteristics of the jaboncillo are discussed in detail by Gutie´rrez-Castorena et al.
Table 3 Percentage of the total mass of the silica features, extracted with repeated NaOH 0.5 N treatments Silica features
Silica nodules Fossilized roots Volcanic glass – not present.
Silica extracted (%) 1
2
3
4
5
6
7
8
9
10
Total
8.4 6.4 14.9
10.8 21.3 6.5
7.7 11.3 10.0
36.5 9.1 8.9
31.2 7.2 8.7
5.4 12.0 3.5
– 6.0 7.5
– 4.2 5.4
– 4.3 3.2
– 5.2 2.4
100.0 87.0 71.0
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7
4
0
6
95
Profile 10
14
17
Depth (cm)
Symbols Ooliths
25
Ooliths with Silica Jaboncillo with mineral grains
50
Jaboncillo Duripan Infilling
75
few
Amorphous silica coatings common abundant
100
few
Silicified roots common
abundant
125
150
175
200
Fig. 3. Distribution of the opaline features in the different horizons and layers in the profiles.
groundmass contains many primary mineral grains (Fig. 5b) forming an open, double-spaced or sometimes even single spaced porphyric c/f related distribution pattern. The coarse fraction consists of volcanic glass (9 to 94%; with a average 42%), and euhedral or subhedral grains of feldspar (anorthite), olivine, hornblende and augite (Table 2). 3.3. White opaline nodules White opaline nodules are only identified in the lacustrine sediments of the central part of the lake and are always related to a jaboncillo groundmass, where they occupy up to 10% by volume. The white
nodules have a diameter of 1 to 2 cm, an irregular or lenticular shape, a soft and gelatinous touch when moist, but hard when dry, and show a banded distribution. In thin sections they are white or light brown in transmitted light and cloudy in oblique incident light (Table 4; Fig. 6a and b). SEM and EDXRA studies show that the nodules consist of an agglomeration of spheres of about 1 Am in diameter of pure opal. This composition is confirmed by XRD (Fig. 7) showing weak reflections of cristobalite and tridymite and a dominance of opal-A and by the fact that after six treatments with 0.5 N NaOH the (hard) nodules completely dissolve, releasing 100% of silica (Table 3).
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Table 4 Optical properties of opaline features Feature
Colour PPL
Limpidity PPL
Relief PPL
OIL
XPL
Ooliths Jaboncillo White nodules Roots Channel coating Fissure coating Infillings (dense complete o incomplete)
Yellow-brown Light brown White, light brown o brown Light brown, brown Light brown Light brown Yellow-brown
Limpid Speckled Limpid Limpid Limpid Limpid, speckled Limpid, speckled
negative negative negative negative negative negative negative
Cloudy Cloudy, light grey Cloudy, light grey Light grey Light grey Yellowish brown Cloudy, light grey
Iso Iso Iso Iso Iso Iso-aniso Iso-aniso
PPL: plain polarised light; OIL: oblique incident light; XPL: crossed polarised; iso: isotropic; aniso: anisotropic.
3.4. Silicified roots Silicified roots occur in the lower parts of the hill (Profiles 6 and 10) or the borders of the lake (Profile 14) and form superposed strata (Table 6) which vary in thickness from 58 to 110 cm. The silicified roots occupy 20 to 70% of the volume of the layer. Their vertical orientation (Fig. 8a) indicates that they have not been hindered in their growth and that therefore the cemented layer they traverse can only have hardened after their formation. The roots can be related to aquatic grasses because of the presence of abundant diatoms. The silicified roots are light grey (10YR 7 / 2), 0.5 mm in diameter, 2 to 7 cm long, fibrous and hard when dry but soft when moist. In thin section the silicified roots appear as series of articulated, although separated, phytholiths in their original position. All cells are prolate, isotropic and perfectly preserved, the silica completely filling their lumen (Fig. 8b). SEM studies show that in most cases it is possible to distinguish the epidermis, the parenchyma, the fibres and the vascular tissues. The poly-
hedral parenchyma cells are characteristic for grasses (Geis, 1978), although also tubular ones occur, as shown in Fig. 8c and d. Microprobe analyses show that the silicified roots are 100% silica. Similar data of 99.9% have been reported for the inorganic parts of bamboo (Jones et al., 1964). The silicified roots dissolve only partially after 10 treatments with 0.5 N NaOH, compared to six treatments for a complete dissolution of the white nodules (Table 3). The larger internal specific surface of the nodules, composed of spheres of about 1 Am may play a role in this difference. These findings also support the idea of Drees et al. (1989) that biogenic and phytogenic opal is less soluble than inorganic opal. According to XRD (Table 5; Fig. 9) only traces of tridymite and cristobalite are present, pointing to an opal-A. In the roots under study all cellulose and lignin has been replaced by silica. The process was rather rapid, as the sediments are less than 1000 years old, based in the presence of underlying buried prehispanic constructions.
Table 5 Mineralogical and chemical composition of opaline features by X-ray diffraction and EDXRA Feature
Amorphous
Cristobalite
Tridymite
Others
EDXRA
Ooliths Jaboncillo White nodules Silicified roots Channel coating Fissure coat Infillings
x xxx xxx xxx xx xx xx
x xx x x xx xx xx
x – x x – – –
Cal. (xxx) Fd., Go., Lp., He., Cal., Do. – – – – –
Ca, Si, Al, Mg Si, Al, Mg, Na, K, Fe, Cu Si Si Si, Ca Si, Ca Si, Ca
Go: goethite; Lp: lepidocrocite; He: hematite; Cal: calcite; Do: dolomite; Fd: feldspars. Relative peak size: xxx, large; xx, medium; x, small; (–) no peaks.
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Fig. 4. Micrograph of concentric calcitic opaline ooliths in thin section, (o) opal, (cc) calcium carbonate; profile 6, 2CBk. a) PPL, b) XPL, frame length 2.9 mm; c) BSI of oolith in thin section with position of line scan; d) line scan for Si, Ca and Al.
3.5. Opaline coatings related to channels Microlaminated isotropic coatings occur around the channels related to the silicified roots (Fig. 8a and b). They are limpid (Table 4) light brown coatings, which have sharp boundaries with the groundmass. The microlaminations are oriented parallel to
the channel wall, indicating a sequential deposition with approximately regular thickness; they are common (Profiles 4 and 6) to abundant (Profile 10) with a random basic distribution. According to XRD (Table 5) cristobalite and opal-A are the main constituents. The channels are surrounded by diffuse iron and/or manganese hydroxide hypocoatings.
Fig. 5. a) Micrograph of jaboncillo: isotropic groundmass and fragment of ostracods in XPL, frame length 5.3 mm, Profile 14, 6Ck; b) dense complete infilling with opal, XPL, frame length 5.3 mm, Profile 14, 2CBk.
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Fig. 6. Opal nodules in jaboncillo of the central part of the lake. a) Lenticular nodule in oblique incident light; b) other nodule in XLP. Frame length 2.9 mm.
cm high) where opal and clay infilling form mounds, up to 10 cm high. These materials are not indurated, but soft with a slippery consistency when moist, and can easily be separated. Micromorphological studies show that these (2.5Y 7 / 6) macro-infillings consist of yellowish brown, isotropic or weakly anisotropic, 300 to 500 Am thick microlaminated coatings (Table 4), indicating a sequential deposition (Fig. 10a, b and c). Most dominant are the isotropic coatings, and those
3.6. Opaline coatings related to fissures In the lower parts of the profiles on the shoulder and foot slope (Profile 4 and 6), opal forms coatings, up to 2 mm thick and 30 cm long. They are mainly related to horizontal fissures between layers and vertical fissures inside a petrocalcic horizon (Profile 10, from 2CBkm to 5Cqk). In the deepest part (160 cm) of Profile 10 a fissure is connected to a crotovina (25 cm wide and 20
4.3
3.8
a 4.1* 4.04** 3.8* 4.3*
3.1**
5
10
15
20
25
30
b
2.4**
2.9*
35
40
Degrees 2 Fig. 7. XRD diffraction. a) silica nodule, b) fossilized root.
45 * Tridymite ** Cristobalite
50
M.C. Gutie´rrez-Castorena et al. / Geoderma 132 (2006) 89–104 Table 6 Presence of silicified roots layers and their thickness in the studied profiles Profile number
Number of root layers
Cumulated thickness of root layers in cm
7 4 6 10 14 17
– – 5 5 4 3
– – 105 110 99 58
showing randomly oriented, weakly anisotropic domains. Less commonly the anisotropism occurs as continuous streaks parallel to the walls, and with a length slow orientation. Between crossed polarizers and with the help of a retardation plate, a moderate cross-juxtaposition can be noticed. In oblique incident light these coatings can be distinguished from those around channels by their yellowish brown colour, compared to the pale yellow colour in the channel coatings, and the presence of anisotropic domains and streaks. The opaline coatings are a mixture of opal-A and cristobalite (Table 5). 3.7. Microlaminated infillings between minerals and ooliths Laminated infillings occur in the soils on the lower parts of the hill (Profiles 6 and 10). They are related morphologically to the coatings and infillings of fissures, as observed in the field and described above, but on a much smaller scale, and only visible at close observation. The opal-A and cristobalite (Table 5) forms microlaminated coatings and infillings in fissures (macro-features), interstices between calcite nodules (described earlier in this paper) and minerals of the sandy layers in profiles 6 and 10, creating a close porphyric c/f related distribution pattern. They are 1 to 2 mm thick and composed of 50 to 150 Am thick microlaminations, comparable to those described for fissures (Figs. 11 and 12). 3.8. Other siliceous components Volcanic glass (Table 2) is part of the sedimentary units (alluvial and lacustrine). It is the principal
99
constituent of the rocks in the surrounding mountains (Rodrı´guez et al., 1999) and was transported by rivers or wind. Residues of diatoms and phytoliths are also abundant, especially in the footslope (Profiles 14 and 17). The phytoliths have polyhedral forms, as common for grasses (Geis, 1978) and they are similar to those observed in the fossilised roots. Although phytoliths of a forest environment have been reported by Rodrı´guez (1999) in the mountainous zone, they were probably not transported to the lake.
4. Discussion The sediments of the former Texcoco Lake, and the soils formed on them are rich in siliceous features of different morphologies and different origins. Some were formed in the highly alkaline former Texcoco Lake previous to its drainage (Del Valle, 1983), other after emergence. The former lake was situated in a closed basin, surrounded by rocks with a high content of acid volcanic glass (70%), which upon weathering released amorphous silica (Rodrı´guez et al., 1999). Amorphous silica concentration in the soils increase from 6% in the surrounding mountains (Segura et al., 2000) up to 45% in the lake (Del Valle, 1983). The global geochemical landscape is characterised by an accumulation of Al in the higher, moister forest soils (e.g. as kaolinite), a retention of Mg in the smectitic soils on the lower, dryer slopes (Segura et al., 2000), and an accumulation of Si, Ca and Na in the basin lake (Del Valle, 1983). 4.1. Ooliths Because of their stratigraphic position these calcitic-opaline features with amorphous silica can be related to the beaches of the former lake (Gutie´rez Castorena et al., 1998). They most probably formed during the last 1000 years as can be deduced from the presence of artefacts, which also proves their sedimentary origin. The area was characterised by an intense evaporation (Sanders, 1983) and a high biological activity (presence of silicified aquatic grasses). In lacustrine environments the evaporation and photosynthesis of aquatic plants, which removes CO2, promotes the precipitation of CaCO3 (Bathurst,
100
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Fig. 8. Silicified roots (Profile 14, 4Ck). a) Soil profile; b) thin section of transversal section through silicified root. PPL, frame length 5.3 mm; c) and d) SEM images showing cells of parenchyma (p), fibers (f) and xylem (x).
1975). Concentric calcite nodules, called ooliths, are formed in shallow agitated water, and are generally associated with micro-organisms (Drees and Wilding, 1987). A decrease of the pH, provoked by the input of pluvial water in the alkaline lake, may be responsible for the precipitation of Si (Siever, 1957; Jones et al., 1967). The repeated input of fresh water, followed by evaporation and biological activity can explain the alternation of sharp bounded calcitic and siliceous laminae. A diagenetic transformation of opal-A may
have been taken place by the presence of cristobalite and tridymite (Table 5). 4.2. Jaboncillo According to Gutie´rrez-Castorena et al. (2005) the jaboncillo is composed of amorphous material of colloidal size that precipitated during the major extension of the lake as a result of small environmental changes (slight evaporation and moderate pH changes
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101
Fig. 9. Coating of microlaminated opal in channel. a) PPL; b) XPL. Frame length 2.9 mm.
to less alkaline conditions). The thin discontinuous lacustrine alluvial layers at the footslope, having the same characteristics as the jaboncillo, were most probably formed under similar conditions. It is clearly a mixture of opal-A with amorphous iron (hydr)oxides and mineral particles of colloidal size. The lower water retention capacity of the jaboncillo in Atenco (at the border of the lake) compared to that in the centre of the lake, points to a relative irreversible dehydration of the sediments at the lake’s edge.
4.3. Nodules White nodules occur in the original sediment as soft, whitish concentrations of silica, pointing to an early diagenetic process of Si accumulation. There is no morphological indication in the surrounding sediment that they could be synsedimentary. They seem to be composed essentially of opal-A. The hardening may be explained by a loss of water resulting in a collapse of the randomly packed opaline spheres giv-
Fig. 10. a) Fragment of coating of opal (o) with fissures infilled by calcite (cc), PPL, frame length 2.9 mm; b) same, but XPL; c) microlaminated coating of clay and opal infilling of crotovina, PPL, frame length 5.3 mm. Profile 10, 5Cqk.
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Fig. 11. a) Dense complete infilling of opal between ooliths. PPL, frame length 2.9 mm; b) same but XPL. Profile 6, Bkqm.
ing rise to closer packing. As they harden upon drying, the presence of soft nodules in the field proves that the layers were never exposed. 4.4. Silicified roots Silicified roots were formed at several intervals, partly in historic times in the lake, pointing to a frequent change in shoreline position. It is an indication for the stability of biogenic opal-A that they are not yet transforming to opal-CT, although younger features, such as pedogenic coatings, are already in that stage of evolution. The fact that numerous similar phytoliths are found in the surrounding material indicates that silicification took place at least partially in the living plant. It has been reported that plants impregnate their epidermis with silica in order to reduce losses of water by transpiration (Raven, 1983) and/or to reduce damage by salinity (Ahmad et al., 1992). Silica is absorbed by
a
the plants as H4SiO4 (Alexander et al., 1954) and transported through the xylem. It can however not be excluded that after burial more silica precipitated from the environment. The formation of microlaminated coatings in the root channels prove that silicification took place before the period of high silica mobility after emergence of the sediments. 4.5. Opaline coatings on channels and fissures The distribution of coatings and infillings is clearly related to pedogenic porosity (shrinkage cracks, biopores), which can only be formed after emergence of the sediments. As the surrounding material seems unweathered, as evidenced by the presence of high quantities of volcanic glass and euhedral olivine (Table 2) the origin of the amorphous silica must be related to the lacustrine jaboncillo layers. An alkalisation of the sediment near the surface after the evaporation of the lake may have caused a solubilisation of
b
Fig. 12. a) Dense complete opal of packing voids, PPL, frame length 5.3 mm; b) same, but XPL. Profile 10, 2CBkqm.
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the silica which, after transport to deeper parts of the soil profiles has precipitated in the zones with a lower pH, and moreover started to crystallise as cristobalite, explaining partially the weak anisotropy (Flach et al., 1969). At the summit position (Profile 7) a strong alkaline pH is measured in which silica is more soluble, and low SiO2/Al2O3 molar ratios (from 5.8 to 24.2), whereas at the base the pH is only slightly alkaline and the SiO2/Al2O3 molar ratios are much higher (up to 199.2) (Profile 10) (Table 1). A transport of Si from the upper part of the lower profile might explain the opal infillings and coatings of the horizontal fissures and the intense accumulation in the lower part, including the infilling of the crotovina. The coatings resemble part of the bmilky pedofeaturesQ described by Usai and Dalrymply (2003). The weak anisotropism can be related to an initial crystallisation of the opal-A to opal-CT (anisotropic domains), or to the presence of fine clay particles oriented according to the flow direction of the colloids (parallel anisotropic streaks). The cross-juxtaposition of the laminae, and the alternation of limpid isotropic and less limpid, weakly anisotropic laminae, points to a complicated, sequential illuviation of silica, maybe partially as solution, partially as colloid. 4.6. Opal infillings In the oolithic layers two generations of opal are present: in the nodules, where opal and calcite accumulated alternatively in the same feature as a result of environmental changes in the lake, and as microlaminated coatings formed by sequential deposition of silica between the coarse elements by pedogenic processes, as described above. The silica precipitates first around the ooliths and mineral grains and later these coatings serve as a base for further deposition creating locally a close porphyric c/f related distribution pattern as mentioned by Chadwick et al. (1987), who describe infillings of up to 150 Am thick in soils with a sandy texture. In the studied soils however the infillings are much larger and related to macropores such as fissures and packing voids. The siliceous coatings and infillings form a horizon with the properties of a duripan (Soil Survey Staff, 1999). It is indurated and slakes in concentrated KOH. These horizons normally form in xeric and arid
103
moisture regimes or in volcanic soils where weathering processes dissolve volcanic glass and liberate soluble silica which precipitate at a given depth. In this case the bduripanQ is formed in an ustic moisture regime and the soluble silica originates from lacustrine sediments rich in this element by lateral transport. The alkaline pH provoked the resolubilisation and the natural relief the redistribution along the slope. As the roots of aquatic plants traverse vertically these cemented layers, it is evident that the duripan postdates the lacustrine environment and therefore must be probably less than 100 years old.
5. Conclusions The Ex-Lago de Texcoco was situated in a closed basin composed of igneous rocks with a high amount of acid volcanic glass (up to 70%), which upon weathering released silica that was transported to the lake and became saturated with this element. In the deepest parts of the lake the silica was precipitated in colloidal form due to small changes in the environment (pH and evaporation) giving rise to an optically isotropic material called bjaboncilloQ. This material is also found in the deepest horizons of the studied soils, closer to the shore, decreasing in thickness towards the summit. Diagenetic concentrations of pure silica formed in the jaboncillo as soft nodules of opal-A, hardening irreversibly upon drying. In old beach ridges, changes in pH, in evaporation and in biological activity lead to sequential deposition of calcium carbonate and opal, forming concentric ooliths. Aquatic grasses whose roots penetrated the lacustrine sediments rich in amorphous silica invaded the borders. The plants absorbed significant amounts of Si and were silicified as opal-A. This process took place during different periods as different superposed strata were formed, partly younger than some prehispanic constructions. Once the sediments emerged, either by natural or by artificial processes, they gave rise to materials with a strong alkaline pH, allowing the Si to go into solution and to migrate to deeper horizons or to move laterally, concentrating in the soils of the lower part of the hill slope, where polygenetic opaline coatings and infillings were formed in pedogenic pore spaces, locally forming a duripan.
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