Silicon Isotopes Record Dissolution and Re-Precipitation of Pedogenic Clay Minerals in a Podzolic Soil Chronosequence

Geoderma 235–236 (2014) 19–29

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Silicon isotopes record dissolution and re-precipitation of pedogenic clay minerals in a podzolic soil chronosequence

Jean-Thomas Cornelis a,b,⁎, Dominique Weis a, Les Lavkulich c, Marie-Liesse Vermeire b, Bruno Delvaux b,JaneBarlinga,1 a Paciﬁc Centre for Isotopic and Geochemical Research, Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia (UBC), 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada b Soil Science and Environment Geochemistry, Earth and Life Institute, Université catholique de Louvain, Croix du Sud 2/L7.05.10, B-1348 Louvain-la-Neuve, Belgium c Soil Science, University of British Columbia (UBC), 127-2357 Main Mall, Vancouver, BC V6T 1Z4, Canada article info abstract

Article history: By providing the largest part of the reactive surface area of soils, secondary minerals play a major role in terres- Received 8 February 2014 trial biogeochemical processes. The understanding of the mechanisms governing neo(trans-)formation of pedo- Received in revised form 20 June 2014 genic clay minerals in soils is therefore of the utmost importance to learn how soils evolve and impact the Accepted 22 June 2014 chemistry of elements in terrestrial environments. Soil-forming processes governing the evolution of secondary Available online xxxx aluminosilicates in Podzols are however still not fully understood. The evolution of silicon (Si) isotope signature in the clay fraction of a podzolic soil chronosequence can provide new insight into these processes, enabling to Keywords: Podzol trace the source of Si in secondary aluminosilicates during podzol-forming processes characterized by the mobi- Silicon isotopes lization, transport and precipitation of carbon, metals and Si. The Si isotope compositions in the clay fraction Soil formation (comprised of primary and secondary minerals) document an increasing light 28Si enrichment and depletion Clay minerals with soil age, respectively in illuvial B horizons and eluvial E horizon. The mass balance approach demonstrates Biogeochemical cycles that secondary minerals in the topsoil eluvial E horizons are isotopically heavier with δ30Si values increasing from −0.39 to +0.64‰ in c.a. 200 years, while secondary minerals in the illuvial Bhs horizon are isotopically lighter (δ30Si = −2.31‰), compared to the original “unweathered” secondary minerals in BC horizon (δ30Si = −1.40‰). The evolution of Si isotope signatures is explained by the dissolution of pedogenic clay minerals in the topsoil, which is a source of light 28Si for the re-precipitation of new clay minerals in the subsoil. This provides consistent evidence that in strong weathering environment such as encountered in Podzols, Si released from secondary minerals is partially used to form “tertiary clay minerals” over very short time scales (ca. 300 years). Our dataset demonstrates the usefulness to measure Si isotope signatures in the clay fraction to discern clay mineral changes (e.g., neoformation versus solid state transformation) during soil evolution. This offers new opportunity to better understand clay mineral genesis under environmental changes, and the short-term impact of the dissolution and re-precipitation of pedogenic clay minerals on soil fertility, soil carbon budget and elemental cycles in soil–plant systems. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Chorover, 2001). The secondary minerals consist of layer-type aluminosilicates (called pedogenic clay minerals) and Fe-, and Al- Soil is a precious but threatened resource (Banwart, 2011). In order oxyhydroxides, both of which play a major role not only in soil fertility, to protect it for the future we need a better understanding of the soil- but also in the transfer of elements and pollutants from land to ocean forming processes controlling the evolution of newly-formed minerals given their high surface reactivity (Sposito, 2008). Moreover, the (secondary minerals). Soil formation progressively modiﬁes parent capacity of charged mineral surfaces to form adsorption complexes rock material and controls the pathways of primary mineral weathering can stabilize organic carbon (OC) in soils through the formation of and secondary mineral synthesis in the clay fraction (Chadwick and organo-mineral associations, partly controlling global C budget (Parﬁtt et al., 1997; Torn et al., 1997). The formation of secondary minerals and their evolution during pe- ⁎ Corresponding author at: Earth and Life Institute (ELI-e), Université catholique de dogenesis have been studied for over a half century (Wilson, 1999). The Louvain (UCL), Croix du Sud 2, L7.05.10, 1348 Louvain-la-Neuve, Belgium. proportion and the chemistry of minerals in the clay fraction change E-mail address: [email protected] (J.-T. Cornelis). 1 Now at Department of Earth Sciences, University of Oxford, South Parks Road, Oxford with soil evolution (Egli et al., 2002; Righi et al., 1999; Turpault et al., OX1 3AN, United Kingdom. 2008). Some environmental changes (vegetation type, agricultural

http://dx.doi.org/10.1016/j.geoderma.2014.06.023 0016-7061/© 2014 Elsevier B.V. All rights reserved. 20 J.-T. Cornelis et al. / Geoderma 235–236 (2014) 19–29 practices, land-use, climate and drainage) can amplify the modification sequence of four soil profiles undergoing podzolization (Cox Bay on of clay mineralogy on very short time-scales (10–1000 years) (Caner Vancouver Island, Canada) (Fig. 1) and for a single Podzol pedon et al., 2010; Collignon et al., 2012; Cornu et al., 2012; Mareschal et al., (Gaume, Belgium). The Cox Bay chronosequence offers an opportunity 2013). These rapid clay modifications occur in chemically reactive soil to study the variation of Si isotopic composition and elemental ratios micro-environments, i.e. the part of the soil influenced by roots and of the clay fraction in the vertical pedogenic scale: E, Bh, Bhs, Bs, Bw earthworms (Calvaruso et al., 2009; Jouquet et al., 2007), and can play and BC horizons, and in the horizontal time-dependent scale: duration a key role in geochemical balance of several minor and major elements of pedogenesis from 0 to 335 years. We used the Belgian Podzol as a in soils and sediments (Michalopoulos and Aller, 1995; Velde and “natural duplicate” in temperate climate to corroborate the processes Meunier, 2008). However, the origin of elements involved in clay documented in the soil samples from the Cox Bay podzolic soil neo(trans-)formation is still not well understood. chronosequence. Podzol, the focus of this study, is a type of soil that covers more than 3% of the Earth's land surface. The low stock of weatherable minerals, the acidic conditions and complexing capacity of organic acids in the en- 2. Materials and methods vironment where Podzols developed are responsible for mobilization, transport and precipitation of carbon (C), metals (Fe, Al) and silicon 2.1. Sample collection and location (Si) in the soil profile (Lundström et al., 2000). A fully developed Podzol consists of a leached gray subsurface eluvial E horizon contrasting with We sampled a soil chronosequence undergoing podzolization in Cox the accumulation of elements in the dark illuvial B horizons. The topsoil Bay (CB), on the west coast of Vancouver Island (British Columbia, is characterized by the production of organic acids that form soluble Canada). At the Cox Bay study site, three main vegetative associations organo-metallic complexes enhancing weathering in the eluvial E hori- are identified in the chronosequence. These correspond to Sitka spruce zon. This E horizon overlies the dark C-enriched Bh horizon and reddish (Picea sitchensis) in the younger site (CB-120 years), and Sitka spruce Fe-, Si-, and Al-enriched Bhs/Bs horizons (Lundström et al., 2000). Given (P. sitchensis) and salal (Gaultheria shallon) in the sites of 175 and the very acidic conditions in Podzols, besides the weathering of primary 270 years (CB-175 and -270 years). The oldest site (CB-335 years) is minerals, secondary clay minerals can be dissolved in the podzolic characterized by Sitka spruce (P. sitchensis), Douglas fir(Pseudotsuga weathering front (Ugolini and Dahlgren, 1987; Zabowski and Ugolini, menziesii), salal (G. shallon) and western sword fern (Polystichum 1992), which describes the soil depth where minerals dissolve faster munitum). Heavy mean annual precipitation (3200 mm) coupled with than they form. A podzolic soil chronosequence, i.e. in which all soil- frequent fogs and sea sprays ensure an abundance of moisture and nu- forming factors remain constant except time; represents an ideal natu- trients year round in this maritime temperate climate (Cfb: without dry ral system for the study of the effect of time on pedogenic clay minerals season and with warm summer; Peel et al., 2007). The Tofino Area behavior in soils. Greywacke Unit is the source of the beach sand parent material, from Stable Si isotopes fractionate during silicate weathering and the bio- which soils have developed in the age sequence (Singleton and geochemical Si cycling (Opfergelt et al., 2010; Ziegler et al., 2005), and Lavkulich, 1987). Sampling sites were located along a transect (0–94 as such provide a means of tracing the bio-physico-chemical processes m) perpendicular to the present shoreline (Fig. 1). Dendrochronology in terrestrial environments (Cornelis et al., 2011). In addition to its in- and geomorphology established surface duration of pedogenesis rang- corporation in the mineral structure during the formation of crystalline ing from 0 to 335 years for the four selected pedons. Tree ages were de- layer-type aluminosilicates, poorly-crystalline aluminosilicates and termined counting the tree rings in the increment bores. Assuming that pedogenic opal, monosilicic acid (H4SiO4) released into soil solution the beach built towards the ocean in a configuration parallel to the can also be transferred into the biosphere to produce biogenic opal existing shoreline and that a linear deposition rate occurred with time (phytoliths) or be adsorbed onto secondary Fe oxy-hydroxides. The in- between successive oldest trees, the rate of advance of the beach front corporation of Si in mineral structures through neoformation of second- was estimated to be 0.26 m per year. At this rate, the 13-m strip of ary pedogenic and biogenic precipitates and its adsorption onto the sand containing tree seedlings would have accumulated in approxi- surfaces of Fe oxides are two processes favoring the retention of light mately 50 years (Singleton and Lavkulich, 1987). With soil develop- 28Si in soils and contributing to the enrichment of rivers in heavy 30Si ment, there was progressive deepening and differentiation of genetic (Delstanche et al., 2009; Georg et al., 2007; Opfergelt et al., 2006; horizons during podzolization, resulting in soil classification (World Ziegler et al., 2005). Clay minerals can also be unstable in organic and in- Reference Base for Soil Resources — WRB) that ranged from Dystric organic acidic environments where they dissolve (Sokolova, 2013; Cambisol at the youngest sites (CB-120 years; CB-175 years) to a Placic Zabowski and Ugolini, 1992), and enrich soil solutions (Cornelis et al., Podzol at the oldest site (CB-335 years) (Fig. 1). The 335-year-old Pod- 2010) and rivers (Cardinal et al., 2010) in light 28Si. The naturally occur- zol is characterized by the following soil horizon development: eluvial ring mass-dependent Si isotopic fractionation is induced by dissolution, albic E horizon (strongly weathered horizon) → illuvial spodic Bh hori- precipitation and adsorption but not by complexation as chemical bind- zon (enriched in organic matter) → Bhs horizon (enriched in Fe ing of Si to organic matter is negligible (Pokrovski and Schott, 1998). It oxyhydroxides and organic matter) → Bs horizon (enriched in poorly- has also been demonstrated that the Si isotopic compositions of second- crystalline aluminosilicates and Fe oxyhydroxides) → Bw horizon ary clay minerals relates to climatic gradient and its control on clay min- (development of color and structure without illuvial accumulation of eralogy (Opfergelt et al., 2012). However Si isotopes have never been materials) → BC horizon (weakly colored and structured; little affected used to better understand clay mineral modifications induced by soil- by pedogenic processes). forming processes under identical geo-climatic conditions. The rapid The sampling area of the Podzol in Gaume (Belgium), ranging in al- modification of clay mineralogy in Podzol is well documented (Caner titude from 300 to 350 m above sea level, has an annual rainfall of et al., 2010; Egli et al., 2002; Righi et al., 1999), but the fate of Si released 1100 mm and a mean annual temperature of 7.7 °C (Herbauts, 1982), in soil solution after clay modification has not yet been studied, even and is also characterized by a maritime temperate climate (Cfb; Peel though it is of crucial importance for identifying the sources controlling et al., 2007). The Podzol is located on the Lower Lias outcrop in South- the formation of pedogenic clay minerals in soils. east Belgium (Gaume). The bedrock (calcareous sandstone of Lower In this study, we aim to use Si isotope signatures of the clay fraction Lias age) is covered by a two-layered sheet: an autochthonous sandy in a podzolic soil chronosequence for gaining better insights into the layer, formed by the dissolution of the calcareous bedrock, is overlaid origin of Si in pedogenic clay minerals. by a mixture of this sandy material with loessic silt-sized particles. To achieve this goal, we analyzed Si isotopes, elemental (Ge/Si, Al/Si, The Belgian Podzol developed under heather (Calluna vulgaris)ischar- Fe/Si) ratios and determined clay fraction mineralogy for an age acterized by a similar morphological profile as the Podzol in Cox Bay J.-T. Cornelis et al. / Geoderma 235–236 (2014) 19–29 21 Vertical scale (m) 50

20 0,26 m/year 10

Beach 0

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Horizontal scale (m) CB-0 yr CB-120 yrs CB-175 yrs CB-270 yrs CB-335 yrs 0 E E E Bh Bh Bw Bhs Bs C BC Bw Bw

BC BC BC -0.75 m Parent Dystric Cambisol Haplic Podzol Placic Podzol material

Fig. 1. Cross section of the Cox Bay study area showing site locations and soil horizons, depending on their respective age of soil formation: CB-0 year, CB-120 years, CB-175 years, CB-270 years and CB-335 years. sequence (CB-335 years) with the following horizons: E–Bh–Bhs–Bs– citrate–bicarbonate, eight standard treatments were applied to deter- Bw–BC. mine mineralogy of the clay fraction: K-saturation (KCl 1 N) followed by drying and heating at 20, 105, 300 and 550 °C, and Mg-saturation

2.2. Physico-chemical characterizations (MgCl2 1 N) followed by drying at 20 °C and saturation with ethylene- glycol (eg). XRD analysis was also performed on powder samples of The soil samples were air-dried, then sieved and homogenized. The the clay-sized fraction after removal of organic matter and Fe content of free iron oxides was assessed after selective dissolution of oxyhydroxides but without any further treatment for quantifying min- Fe oxides using Na-dithionite–citrate–bicarbonate and ammonium oxa- eralogy of the clay fraction using the Siroquant software V4.0 (Sietronics late–oxalic acid (Fedcb = crystalline Fe oxides, Feox = poorly-crystalline Pty Ltd), and for the following horizons: BC horizon (CB-120 years), E Fe oxides). The content of Si bound to poorly crystalline aluminosilicates horizons (CB-175, 270 and 335 years), Bh horizon (CB-335 years) and and weakly-ordered Fe oxyhydroxides was estimated on fine earth by Bhs horizon (CB-335 years). extraction with ammonium oxalate–oxalic acid (Siox). Al complexed with organic ligands was assessed using the complexing agent Na- 2.4. Isotopic and geochemical analyses pyrophosphate at pH 10 (Alp). The total organic carbon (OCtot) content was measured on ground samples using CNS analyzer. Silicon isotope compositions and elemental (Ge, Al, Fe and Si) con- The clay fraction (b2 μm) was separated using a ‘clean procedure’ centrations were measured on clay-sized fraction (b2 μm) extracted without any oxidative treatment. Air-dried soil was dispersed in deion- from all the horizons of the four soil profiles in Cox Bay (clay-CB ized water and sonicated. The suspension was then separated on a 120 years; clay-CB 175 years; clay-CB 270 years; clay-CB 335 years) 50 μm sieve, re-suspended in deionized water and sonicated and sieved and the undated podzolic soil profile in Gaume (clay-G), and also on until the supernatant was clear after sonication. The fraction retained in parent material of soils in Cox Bay (sand fraction of the beach sand; the sieve was collected as the N50 μm sand fraction. Clay (0–2 μm) and Beach-CB 0 year). An alkaline digestion with 99.99% pure NaOH is silt (2–50 μm) fractions were then collected by gravimetric sedimenta- used to transform solid samples into an aqueous HF-free solution tion after dispersion using an ultrasonic probe and Na+ as a dispersion (Georg et al., 2006). All dissolutions and chemical separations were car- agent. ried out in Class 100 laminar flow hoods in Class 1000 clean labs, mass spectrometric analyses were performed in Class 10,000 laboratories at 2.3. X-ray diffraction patterns the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia (UBC). Al, Fe and Si contents of the XRD analyses were carried out on the clay-sized fraction (b2 μm) of dissolved NaOH fusions were analyzed by ICP-OES (Varian 725-ES) soil horizons sampled in the Cox Bay chronosequence (120, 175, 270 with Europium as the internal standard. For Ge measurements, the dis- and 335 years), using CuKα radiation in a Bruker Advance diffractome- solved NaOH fusions were dried and re-dissolved in 1% v/v HNO3 with ter. After removal of the organic matter by treating the sample with 6% 10 ppb indium (In) for analysis by HR-ICP-MS (Element 2) in medium

H2O2 at 50 °C, and removal of Fe-oxyhydroxides using dithionite– resolution. 22 J.-T. Cornelis et al. / Geoderma 235–236 (2014) 19–29

The remaining dissolved NaOH fusion solution was purified for iso- the weathered feldspar. We observe an increase of oxalate-extractable topic analyses through cation exchange chromatography (Georg et al., Siox in Bhs, Bs and Bw horizons of the Podzol (CB-335 years) 2006). The Si isotope compositions were measured on a Nu Plasma (Table 1). We also document a strong mobilization of Fe in Podzol (Nu 021; Nu Instruments Ltd, UK) MC-ICP-MS in dry plasma mode after 335 years, characterized by an accumulation of crystalline and using type B cones and a Cetac Aridus II desolvating nebulizer system. amorphous Fe oxides in the Bhs horizon, which is related to an increase Instrumental mass bias was corrected by simple sample-standard of OC content. This co-accumulation of Fe oxides and OC is also observed bracketing of measured Si isotope ratios, i.e. one sample measurement in the Belgian Podzol in Bh and Bhs horizon. The content of clay-sized normalized to the average of two bracketing NBS-28 standard measure- minerals is quite constant in the Belgian Podzol while we observe an in- ments. Silicon isotopic compositions are expressed as deviations in 30Si/ crease of clay content towards the topsoil in the Canadian podzolic soil 28Si relative to the NBS-28 reference standard using the delta (δ) per mil chronosequence (Table 1). The content of clay-sized minerals in the en- 30 30 28 30 28 (‰) notation: δ Si = [( Si / Sisample)/( Si / SiNBS28) − 1] × 1000. tire soil profiles increases over time in the chronosequence. Each sample was measured at least twice during different analytical ses- The mineralogy of the clay fraction in the Cox Bay podzolic sions. Silicon isotopic (δ30Si) values are reported as the mean of chronosequence is dominated by quartz, amphiboles, chlorites, vermic- replicate isotopic analyses (n N 2) ± 2 standard deviations (SD). The ulite, mixed-layers minerals (MLM), smectite, illite, and kaolinite and NBS-28 (quartz standard) which processed through the full analytical evolves depending on soil age and the development of soil horizons procedure, and analyzed over a period of 7 months during 5 data acqui- (Fig. 2). In the youngest soil profile (CB-120 years), the clay mineralogy sition sessions gave a value of δ30Si = 0.01 ± 0.18‰ (2SD, n = 66). Ac- is characterized by the presence of quartz, Na-feldspar and amphiboles curacy and reproducibility were also checked on reference materials as primary minerals and kaolinite, chlorite and illite as pedogenic clay (diatomite and BHVO-2) at the beginning and at the end of each sample minerals (data not shown). XRD patterns display similar mineral com- series. These gave values identical within error to previously published positions in the E horizons of CB-175 years and CB-270 year profiles. values: 1.24 ± 0.13‰ (2SD, n = 15) for diatomite and −0.29 ± 0.19‰ In those soil horizons, peaks at 1.40, 1.00, 0.83 and 0.70 nm, correspond (2SD, n = 6) for BHVO-2 (Reynolds et al., 2007; Savage et al., 2012). respectively to chlorite, illite, amphibole and kaolinite (disappearance of the 0.7 nm peak after K 550 °C treatment). A band at 1.40 nm 3. Results (Mg 20 °C treatment) that shifts to 1.60–1.70 nm after Mg–eg treatment due to swelling indicates the presence of discrete smectite. In addition, 3.1. Soil mineralogy the combination of the peaks at 1.40 nm after Mg-20 °C and Mg–eg, and the collapse of the peak from 1.10 to 1.00 nm due to the dehydration The parent material of the soil chronosequence (0–335years)isCox after a K-saturation followed by heating correspond to vermiculite. Fi- Bay beach sand (Singleton and Lavkulich, 1987), which is comprised of nally, the presence of a wide peak at 1.20 nm after Mg-20 °C treatment very well-sorted glacial sands with little wearing off and smoothing that shifts after Mg–eg treatment indicates irregularly mixed-layer min- sharp edges and corners. The primary minerals present in the beach erals (MLM). sand C material identified by X-ray diffraction and microscopy are In the CB-335 years profile, mineralogical differences were ob- quartz, amphibole, pyroxene, olivine and feldspars, as well kaolinite served. In the E horizon, relative to the E horizons of CB-175 years and precipitating in the dissolution pits of feldspars. The parent material CB-270 year profiles, XRD patterns show a strong decrease of the abun- does not contain inherited clay minerals, except kaolinite present in dance of kaolinite (the 0.70 nm peak has almost disappeared), absence

Table 1 Summary of the major soil physical and chemical characteristics (for the ﬁne earth b2 mm and the clay fraction b2 µm) of the investigated soils.

a b Horizon Depth pH Soil fractions Siox Sidcb Feox Fedcb Alox Alp OCtot Clay fraction Sand Silt Clay Si Al Fe Ge

cm % g.kg−1 % μg·g−1

Cox Bay 120 years (Dystric Cambisol) BC 0–75 5.9 99.2 0.6 0.3 0.1 0.4 1.7 1.8 0.7 0.5 9.5 15.8 8.6 12.0 2.9

Cox Bay 175 years (Dystric Cambisol) E0–3 5.4 90.2 7.1 2.7 0.1 1.1 1.4 2.8 0.4 0.3 35.2 22.0 8.3 8.1 5.7 Bw 3–44 5.8 99.0 0.6 0.4 0.1 0.3 1.7 2.0 0.9 0.7 4.3 13.5 7.4 10.5 2.0 BC 44–75 5.9 99.6 0.2 0.1 0.1 0.2 1.0 1.3 0.6 0.4 2.7 16.1 8.9 9.4 3.1

Cox Bay 270 years (Haplic Podzol) E0–7 4.6 90.8 6.1 3.1 0.1 0.8 1.2 2.5 0.6 0.4 13.3 23.3 9.1 5.6 8.9 Bh 7–23 5.1 97.2 1.7 1.0 0.2 0.4 2.3 2.6 1.3 0.9 16.1 15.5 8.7 12.1 3.0 Bw 23–57 5.3 97.4 1.8 0.8 0.2 0.4 2.1 2.3 1.1 0.8 10.4 16.1 8.8 12.0 2.8 BC 57–75 5.4 98.2 1.1 0.7 0.2 0.3 1.7 2.2 1.4 1.0 7.6 13.1 10.0 11.3 2.4

Cox Bay 335 years (Placic Podzol) E0–16 4.8 82.3 14.4 3.0 0.1 0.4 0.2 0.5 0.6 0.5 10.7 26.5 10.5 1.9 12.9 Bh 16–23 5.6 88.0 8.7 2.8 0.5 1.0 3.3 4.4 8.8 5.0 36.8 16.9 14.3 6.2 5.8 Bhs 23–24 Nd 90.0 6.8 2.9 1.1 1.6 21.5 44.0 5.2 4.4 17.8 6.2 9.4 30.2 4.6 Bs 24–28 5.1 94.9 4.2 0.9 2.9 1.0 3.7 4.0 7.8 1.3 5.2 14.2 19.6 9.3 3.0 Bw 28–60 5.1 96.1 2.4 1.4 2.5 0.9 2.1 2.9 6.4 1.1 3.8 13.2 20.3 7.5 3.8

Gaume (Haplic Podzol) E19–35 5.0 94.0 3.1 2.9 0.0 0.0 0.1 2.1 0.04 Nd 1.3 12.6 7.6 10.9 2.8 Bh 35–40 4.7 89.0 7.0 4.0 0.1 0.3 4.6 16.7 1.3 Nd 14.4 11.0 7.8 14.5 2.9 Bhs 40–47 4.8 90.0 6.0 4.0 0.3 0.3 5.5 16.8 2.0 Nd 6.1 7.7 9.6 19.7 1.5 Bs 47–58 5.1 91.6 3.4 5.0 0.5 0.4 0.7 6.8 2.2 Nd 3.6 9.9 12.7 12.9 1.9 BC 70–100 4.6 92.9 2.5 4.6 0.2 0.1 0.1 2.8 0.6 Nd 0.7 9.8 13.9 13.8 1.4

Nd = not determined. a Dithionite- (dcb), oxalate- (ox) and pyrophosphate- (p) extractable contents of Fe, Al and Si. b Total organic carbon. J.-T. Cornelis et al. / Geoderma 235–236 (2014) 19–29 23

ABCB-175 yrs : E CB-270 yrs : E

1.39

1.39 0.99 0.83

1.62 Mg eg 0.83 1.19 1.20 0.99 Mg eg

Mg 20°C Mg 20°C

K 550°C K 550°C

K 300°C K 300°C

K 150°C K 150°C

K 20°C K 20°C

4 10 4 10 2 - theta [°] 2 - theta [°]

CDCB-335 yrs : E CB-335 yrs : Bh

1.64 0.70 1.39 1.39 0.99 0.83 0.70

Mg eg 1.19 0.83 0.99

Mg eg Mg 20°C 1.20

K 550°C Mg 20°C

K 550°C

K 300°C K 300°C

K 150°C K 150°C

K 20°C K 20°C

4 10 4 10 2 - theta [°] 2 - theta [°]

Fig. 2. XRD patterns of the clay-sized fraction (b2 μm) of soils of the Cox Bay soil chronosequence after six treatments: K-saturation followed by drying at 20, 105, 300 and 550 °C, and Mg- saturation followed by drying at 20 °C and saturation with ethylene-glycol. (A) CB-175 years E horizon, (B) CB-270 years E horizon, (C) CB-335 years E horizon, (D) CB-335 years Bh horizon. Spacings of major reflections are in nanometers. of chlorite (no peak at 1.40 nm after K treatments), and increase of the the clay fraction in the Belgian Podzol is comprised of vermiculite, smec- relative abundance of smectite compared to vermiculite (increase of the tite, hydroxyl-interlayered vermiculite, chlorite, MLM and kaolinite peak at 1.60–1.70 nm and almost no peak at 1.40 nm after the Mg–eg (Herbauts, 1982). treatment). In the Bh horizon relative to the E horizon of CB-335 year As we are not able to precisely quantify each type of 2:1 minerals on profile, XRD patterns show the presence of kaolinite and chlorite, ab- the powder of the clay fraction (chlorite, smectite, vermiculite, illite, sence of smectite (no swelling after Mg–eg treatment), increase in the MLM) with Siroquant software, we carried out the clay mineralogy abundance of vermiculite, and a decrease of the abundance of MLM quantification in the soil chronosequence by separating the minerals (smaller peak at 1.20 nm after Mg 20 °C treatment). in the clay fraction in 4 groups: quartz, amphiboles, kaolinite The mineralogy of the Belgian Podzol (Gaume) is compared to the and 2:1 minerals (Fig. 3). Compared to the mineralogy of BC horizon mineralogy of the Canadian Podzol. The primary minerals of the loess (CB-120 years) at the initial stage of soil formation (quartz = 15%, contain quartz, feldspars, micas and small amounts of trioctahedral amphiboles = 63%, kaolinite = 5% and 2:1 minerals = 17%), the chlorites and amphiboles (Van Ranst et al., 1982). The mineralogy of quantification of clay mineralogy indicates an increase of the relative 24 J.-T. Cornelis et al. / Geoderma 235–236 (2014) 19–29

30 (at t = 335 years); and ΔSiB–BC varying from −0.17‰ (at t = 175 years) to −0.32‰ (att=335years). A comparable depletion/enrichment in light 28Si in the clay fraction 30 during pedogenesis is found in the Belgian Podzol ( ΔSiE–BC = 30 +0.29‰; ΔSiB–BC = −0.27‰) from a similar temperate climate but with a different parent material and rainfall conditions (Fig. 4D).

3.3. Geochemical modiﬁcations in the clay fraction over time

As the clay fraction becomes relatively more depleted in Si, the clay fraction becomes more enriched in light 28Si (Fig. 5A). Our results show that Si isotopic signature of the clay fraction becomes increasingly light with enrichment in Al (higher Al/Si ratio in the clay fraction) (Fig. 5B). The enrichment in light 28Si (and the increase of Al/Si ratio) in the clay fraction also relates to an increase in the proportion of Fig. 3. Quantitative evolution of the mineralogy in the clay-sized fraction of the Cox Bay soil chronosequence. The clay-size mineralogy is comprised of primary minerals (quartz poorly-crystalline Si components in the clay fraction (estimated by the and amphiboles) and pedogenic clay minerals (kaolinite and 2:1 minerals). Chlorite, Siox/Siclay ratio). As the Si-bearing phases of the clay fraction accumu- vermiculite, smectite, illite and mixed-layer minerals (MLM) are the 2:1 aluminosilicates lates poorly-crystalline aluminosilicates, the Si isotopic composition be- encountered in the podzolic chronosequence. comes more enriched in light Si isotope (Fig. 5C). We observe also that the enrichment in light 28Si in the clay fraction is not systematically related to a relative depletion in Ge, i.e. lower Ge/Si ratio (Fig. 5D). abundance of kaolinite (+14%) and 2:1 minerals (+6%) in E horizon of the 175-year-old soil. Then we observe a strong decrease of the relative abundance of kaolinite in older and more weathered E horizons: −12% 4. Discussion in the 270-year-old soil and −18% in the 335-year-old soil, while the relative abundance of 2:1 minerals is constant between the two oldest 4.1. Evolution of clay-sized mineralogy soils (=18%). In the Bh and Bhs horizons of the 335-year-old soil, we note an increase of the relative abundance of kaolinite (+7% and Different processes, such as transformation and neoformation, mod- +12%, respectively) compared to the stronger weathered E horizon ify the chemical composition of the clay mineral within soil profile and (=1%). The evolution of primary clay-sized minerals is characterized control the clay content and mineralogy during pedogenesis. As water by a decrease of the relative abundance of amphiboles in the early acts to mediate chemical reactions and to transport reactants and prod- stage of soil formation (−21%), then by a relative increase of the abun- ucts from topsoil (Chadwick and Chorover, 2001), we observe the dance (+14 and +19%) in the more weathered E horizons, which is re- highest content of pedogenic subproducts (clay-sized minerals) in the lated to the decrease of kaolinite, while quartz content remains quite top- and subsoils (0–24 cm). The depth where clay-sized minerals con- constant (=16 ± 2%) during pedogenesis. centrate (~3%) increases over time, which highlights the deepening of the weathering front: 0–3 cm after 175 years, 0–7 cm after 270 years, and 0–24 cm after 335 years. We show that the chemical modifications 3.2. Si isotopic modifications in the clay fraction over time of clay mineral structure in the podzolic weathering front mobilize Al (and Fe) and Si from secondary minerals over time. The evolution of In our study, pedogenic clay minerals in the clay fraction of BC hori- Al/Si in the clay fraction substantiates the preferential mobilization of zon are considered as “unweathered” secondary minerals compared to Al, relative to Si, during the dissolution of secondary clay minerals, in pedogenic clay minerals in more weathered horizon (E, Bh, Bhs and particular in the presence of organic acids with high complexing capac- Bs) since BC horizon is not yet reached by the podzolic weathering ities, such as those encountered in Podzols (Sokolova, 2013; Stumm, front (Lundström et al., 2000). In the Cox Bay soil chronosequence, we 1992). The clay mineralogy evolution (Fig. 2) in Podzols studied here therefore compare the Si isotopic signatures of the clay fraction in under maritime temperate climate is very similar to the ones observed each soil horizon with those in the “unweathered” clay fraction in the from postglacial moraines (Righi et al., 1999) and tills (Egli et al., BC pedogenic horizon. 2002) in Switzerland. The aluminization of primary clay minerals, In the Cox Bay chronosequence, Si in the “unweathered” clay such as chlorites, leads to formation of irregularly-interstratified min- fraction (BC horizon; δ30Si = −0.52 ± 0.16‰, 2SD, n = 3) is erals in the moderately acid B horizons. In the stronger weathering E isotopically lighter compared to the primary lithogenic minerals in the system, Al-removal from interlayers by organic complexing agents parent beach sand material (C material; δ30Si = −0.27 ± 0.10‰,2SD, leads to the formation of vermiculite. Further alteration induces the for- n = 3) (Fig. 4A). In the early phase of soil formation, the difference of mation of smectite-like minerals in the E eluvial horizon. Finally, the Siox Si isotope signature between the lithogenic primary minerals in the content (Table 1)confirms that the formation of poorly-crystalline alu- sand fraction of the C material and the clay fraction in BC material, 30ɛ minosilicates (ITM) occurs when the concentration of organic acids is is −0.25‰ (min − max = −0.12 − 0.37‰). This is not the fraction- sufficiently low to allow the precipitation of Al with Si, as suggested ation factor due to precipitation of pedogenic clay minerals as the clay by Ugolini and Dahlgren (1987) in the fulvate bicarbonate theory of fraction also comprises lithogenic primary minerals. podzolization. The clay mineralogy evolves with increasing weathering Relative to the “unweathered” BC clay fraction (δ30Si = −0.52 ± in the age sequence and formation of typical podzolic soil horizons 0.16‰, 2SD, n = 3), the clay fraction of the topsoil eluvial E horizons (E, Bh, Bhs, Bs, Bw), which is in good agreement with the formation of shows depletion in light 28Si (i.e., less negative δ30Si values: from two geochemical compartments during podzolization (Ugolini and −0.33 ± 0.02‰ to −0.10 ± 0.22‰ Fig. 4B, C). The clay fraction in the Sletten, 1991). The upper E-Bh compartment is controlled by organic subsoil illuvial Bh–Bs horizons is isotopically lighter (i.e., enriched in acids as major proton donors and complexing metals, which leads to light 28Si) than “unweathered” BC clay fraction (δ30Si from −0.60 ± dissolution of primary and secondary minerals. In the lower Bhs-Bs 0.06‰ to −0.84 ± 0.08‰‰; Fig. 4B, C). The magnitude of light compartment, the absence of organic acids leads to a less aggressive Si depletion/enrichment in the clay fraction increases with soil age, weathering system mainly controlled by inorganic acids (carbonic and 30 with ΔSiE–BC varying from +0.20‰ (at t = 175 years) to +0.42‰ nitric acids). J.-T. Cornelis et al. / Geoderma 235–236 (2014) 19–29 25

Fig. 4. Silicon isotopic signature (δ30Si ‰; mean values ± standard deviation represented by error bars) in the clay-sized fraction depending on soil ages and in primary lithogenic minerals in the beach sand parental material. (A): 0- and 120-year-old soil fraction (δ30Si of primary minerals in beach sand in black and δ30Si of the clay fraction of the 120-year-old BC horizon in blue), (B): 175- and 270-year-old clay fractions (175 years = red Δ; 270 years = green ◊), (C): 335-year-old clay fraction (purple ○), and (D): clay fraction in an undated Belgian Podzol (brown □). After only 175 years (B), we observe the depletion in light 28Si in the clay fraction of the eluvial E horizon and enrichment in light 28Si in the clay fraction of deeper illuvial soil horizon; respectively, relative depletion in light 28Si (+0.20‰) and relative enrichment in light 28Si (−0.17‰) compared to the original Si isotopic signature of the unweathered clay fraction in the BC horizon. The isotopic fractionation increases over time with an enrichment in heavy 30Si of +0.42‰ in the clay fraction of the E horizon and a concomitant enrichment in light 28Si of −0.32‰ in the clay fraction of the Bhs horizon (after 335 years). We observe exactly the same tendency in the Belgian Podzol with enrichment in light 28Si in the clay fraction of the Bhs horizon of −0.27‰ compared to the unweathered clay fraction in BC horizon. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Four important mineralogical evolutions are observed in the Cox Bay Cameroon weathering sequence varies between 0.08 and 0.45‰ soil chronosequence, as a result of podzolization: (i) the neoformation (Opfergelt et al., 2010). However, all of the Fe in the Cameroon of kaolinite, illite and chlorite from dissolution of primary minerals at weathering sequence is in the clay fraction, while in the temperate the very beginning of soil formation, (ii) the disappearance of kaolinite soils of the Cox Bay chronosequence, only 20% of the bulk Fe content in the strongest weathered E horizon, then (iii) the increase of relative is in the clay fraction for Bhs horizon (=8.8 g·kg−1), where we observe abundance of kaolinite in Bh and Bhs horizons compared to E horizon the largest enrichment in light Si isotope. In eluvial E horizons, we (Fig. 3), and finally (iv) the accumulation of imogolite-type materials observe the largest depletion in light 28Si while the Fe content in in Bhs and Bs horizons (Table 1). the clay fraction represents between 70 and 100% of the total Fe concentration in bulk soil (until 2.8 g·kg−1). The ratio of Fe oxides in 4.2. Dissolution and re-precipitation of pedogenic clay minerals during the clay fraction to Si content in the clay fraction is similar between podzolization Bhs (14%) and E (13%) horizons, while the Si isotope composition in the clay fraction follows opposite trends in these two horizons. As a Since the clay fraction of soils comprises aluminosilicates and Fe-, consequence, we assume that the δ30Si values of the clay fraction of and Al-oxyhydroxides, Si in the clay fraction includes Si incorporated Belgian and Canadian temperate soils can be considered representative in primary minerals (quartz and amphiboles), secondary minerals of the Si isotopic composition of the primary and secondary silicates, (kaolinite and 2:1 minerals) and Si adsorbed onto Fe oxyhydroxides. and not significantly influenced by the fractionation of Si isotopes In the Bhs horizon of the 335-year-old soil, the high content of free Fe through adsorption onto Fe oxides. The role played by the Si adsorption −1 (Fedcb =44g·kg ) is in the same order of magnitude than in a onto Fe oxides on Si isotope compositions of the clay fraction must weathering sequence in Cameroon (20–85 g·kg−1)(Opfergelt et al., however be further investigated. 2009), for which the variations of δ30Si values in the clay fraction due It is well established that the preferential incorporation of light 28Si to adsorption onto Fe oxides are known (Opfergelt et al., 2010). We during neoformation of secondary pedogenic minerals accounts for have to take into account the pool of Si adsorbed onto Fe oxides in the their isotopically lighter signature relative to primary lithogenic min- clay fraction as this Si pool significantly influences the enrichment in erals (Georg et al., 2009; Opfergelt et al., 2010; Ziegler et al., 2005). light 28Si in the clay fraction: the difference of the Si isotope signature The Si isotope composition of the soil clay fraction depends on the de- in the clay fraction of B horizons before and after dithionite-treatment gree of soil weathering and the evolution of the clay mineralogy (i.e., after the release of Si from the surface of Fe oxides) in the (Opfergelt et al., 2010, 2012; Ziegler et al., 2005). 26 J.-T. Cornelis et al. / Geoderma 235–236 (2014) 19–29

Fig. 5. Evolution of Si isotope composition with elemental composition (Si, Al, Ge) and the proportion of poorly crystalline Si (Siox/Siclay) in the clay fraction for the Cox Bay soil chronosequence (175-year-old soil = red Δ; 270-year-old soil = green ◊; 335-year-old soil = purple ○) and for the Gaume Podzol (brown □). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

30 30 30 Using the quantiﬁcation of primary minerals (quartz and amphi- pedogenic minerals ( ε = δ Simin I − δ Simin II) is therefore boles) and secondary minerals (kaolinite and 2:1 minerals) and the Si −1.13‰. The mass balance approach (Table 2) shows also a progressive isotope signature of lithogenic primary minerals (−0.27‰), we can depletion in light 28Si in secondary minerals of the E horizon (from compute δ30Si value of “unweathered” secondary clay minerals in the −0.51‰ to 0.64‰) and an enrichment in light 28Si in secondary min- clay fraction of BC horizon (−1.40‰; Table 2). The isotopic fraction- erals of the illuvial horizons (until −2.31‰). In identical bio-geo- ation factor between primary lithogenic minerals and secondary climatic conditions, the Si isotopic fractionation associated with the

Table 2 Quantiﬁcation of primary and secondary minerals in the clay-sized fraction of the Cox Bay soil chronosequence. The clay-sized quantiﬁcation is then used for the Si isotopic mass balance approach.

Measured data Computed

30 30 30 c Primary minerals Secondary minerals δ Si(‰) δ Si (‰) Δ SiBC–x (‰) (% in the clay fraction)a (% in the clay fraction) in the clay fraction of pedogenic clay mineralsb

BC horizon 78 22 −0.52 −1.40 – (120 years) Ehorizon 59 42 −0.32 −0.39 +1.01 (175 years) Ehorizon 75 25 −0.33 −0.51 +0.89 (270 years) Ehorizon 81 19 −0.10 +0.64 +2.04 (335 years) Bh horizon 72 28 −0.45 −0.92 +0.48 (335 years) Bhs horizon 70 30 −0.84 −2.31 −0.91 (335 years)

a Mineralogy of the clay fraction quantified using the Siroquant software V4.0; primary minerals = quartz + amphiboles; secondary minerals = kaolinite + 2:1 minerals (vermiculite, smectite, illite, chlorite, mixed-layers minerals). b 30 30 30 30 The δ Si of secondary minerals present in the clay fraction is computed as follows: δ Simin II =((δ Siclay fraction − %minI∗ δ Simin I) / % min II), where min I = primary minerals, 30 min II = secondary minerals and δ Simin I = −0.27‰. c 30 30 Si isotope discrimination between “unweathered” clay minerals in BC horizon and pedogenic clay minerals in the “x” horizon of interest (x = E, Bh or Bhs horizons): δ SiE–δ SiBC or 30 30 δ SiBC–δ SiBh/Bhs. J.-T. Cornelis et al. / Geoderma 235–236 (2014) 19–29 27 dissolution of primary lithogenic minerals and neoformation of second- clay minerals discriminate against the release of heavy 30Si as already ary pedogenic minerals should generate comparable Si isotopic signa- demonstrated for diatoms (Demarest et al., 2009) and crystalline basalt tures in the clay fraction in the entire soil profile with no evolution (Ziegler et al., 2005). Besides the lithogenic and biogenic Si pools, we over time given identical fractionation factor between the primary and provide evidence that pedogenic Si pool is therefore involved in the secondary Si pools. Here, we show that that the signature of secondary neoformation of pedogenic clay minerals and as such in the evolution minerals varies in the soil profile and the relative depletion/enrichment of their Si isotope signatures (Fig. 6). in E and B horizons increases with time in the Cox Bay chronosequence. The dissolution of primary minerals and precipitation of secondary min- 4.3. Implications for podzolization theory erals therefore cannot explain the increasing depletion/enrichment in light 28Si in the clay fraction over time and with depth. This highlights For the first time, we document enrichment in light 28Si in secondary that the evolution of δ30Si values in the clay fraction of the soil profiles clay minerals over time in a podzolic soil chronosequence. The highest 28 observed here rules out the weathering of primary minerals (lithogenic enrichment in light Si and oxalate-extractable Siox in Bhs/Bs horizons Si pool) as the sole source for the neoformation of secondary minerals in relative to E/Bh horizons (Fig. 5C; Table 2) highlights that the dissolu- the clay fraction. tion of secondary aluminosilicates in E/Bh horizons acts as a Si source Germanium (Ge), a chemical analog of Si, generally follows similar for formation of poorly-crystalline aluminosilicates (imogolite-type ma- inorganic geochemical pathways than Si (Froelich and Andreae, 1981). terials ITM) in Bhs/Bs horizons. The release of Si from the dissolution of However, secondary pedogenic (clay) and biogenic (phytoliths) min- primary and secondary clay minerals and precipitation of dissolved Si erals display contrasting Ge/Si ratios: neoformed clay minerals are with Al released by microbial decomposition from the organic ligands enriched in Ge (higher Ge/Si) while biogenic silica polymerized in (Lundström et al., 1995) can explain the formation of ITM in Bhs/Bs ho- plants as phytoliths is depleted in Ge (lower Ge/Si) (Derry et al., 2005; rizons (Ugolini and Dahlgren, 1987). During podzol development, ITM Kurtz et al., 2002). Although there is a negative relationship between undergo additional dissolution for the re-precipitating Si as crystalline Ge/Si ratios and δ30Si in the youngest soils (Cambisols) of the Canadian tertiary clay minerals in Bhs horizon. The evolution of Si isotopic signa- soil chronosequence, the absence of a relationship between Ge/Si and ture in pedogenic clay minerals of the podzolic soil chronosequence δ30Si ratios in the oldest soil (Podzol) of the Canadian chronosequence therefore corroborates the process of dissolution and re-precipitation and in the Belgian Podzol (Fig. 5D) allows us to dismiss the dissolution of aluminosilicate phases during podzolization (fulvate bicarbonate the- of phytoliths (biogenic Si pool) as a major source of Si for clay ory; Ugolini and Dahlgren, 1987). We can infer that low contents of neoformation. This process would be characterized by enrichment in poorly-crystalline ITM in the Bhs/Bs horizons play a key role in the evo- light 28Si and depletion in Ge in secondary clay minerals relative to lution of Podzols and the progressive enrichment in light 28Si in pedo- beach sand parent material, as phytoliths are Ge-depleted (low Ge/Si genic clay minerals. The absence of ITM in the Bh horizon and the ratio) relative to primary minerals (Derry et al., 2005). lighter δ30Si in Bhs/Bs indicates their high reactivity during podzoliza- The mass balance approach (Table 2) shows that the enrichment in tion, dissolving as organic-rich Bh horizon forms and precipitating as light 28Si of secondary minerals of Bhs horizon (−2.31‰) compared to Fe-, Si-, and Al-enriched Bhs/Bs horizons form. This is confirmed by the “unweathered” secondary minerals in the BC horizon (−1.40‰) the high reactivity of ITM also reflected in Ge/Si and δ30Si patterns in partly explains the depletion in light 28Si of secondary minerals in the soil solutions of the Santa Cruz soil chronosequence, which indicates clay fraction of E horizon (+0.64‰) for the oldest soil (Podzol CB- seasonal precipitation and dissolution of hydroxyaluminosilicates such 335 years). Our data highlight that the isotopic fractionation due to as allophane (White et al., 2012). The positive correlation between 28 30 preferential release of light Si during dissolution of secondary Siox/Siclay and δ Si values in the clay fraction (Fig. 5C) highlights that 30 minerals in the E horizon (Δ SiE–BC =+2.04‰) partly accounts during podzolization, pedogenic clay minerals become enriched in for the enrichment in light 28Si during re-precipitation of new clay light 28Si together with Al in the poorly-crystalline part of the clay frac- 30 minerals in Bhs horizon (Δ SiBhs–BC = −0.91‰). This combined with tion. Based on these findings, poorly-crystalline aluminosilicates can be the fact that kaolinite is progressively dissolved in the E horizon and regarded as a temporary reactive reservoir of light 28Si in Bs horizon. is almost completely dissolved in the strongly weathered E horizon This reservoir acts as a source of light 28Si in tertiary crystalline clay min- (CB-335 years) (Fig. 3), highlights that 28Si is redistributed in the soil erals, such as tertiary kaolinite, in Bhs horizon that will develop in the profile through re-precipitation of new pedogenic clay minerals deeper current Bs horizon during podzolization. The dissolution and re- in the soil profile and leaching. As a part of Si precipitating during the precipitation of pedogenic clay minerals are therefore an important neoformation comes from the dissolution of secondary clay minerals, podzol-forming process (Fig. 6). we name those new clay minerals as “tertiary minerals”. This implies that the preferential lessivage of clay particles enriched in light 28Si 4.4. Implications for tracing the effects of environmental changes on soils and the resulting relative accumulation of primary clay-sized minerals in topsoil cannot be responsible for the on-going enrichment in light In the Cox Bay soil chronosequence, we show that the production of 28Si in the clay fraction. Indeed, the increasing enrichment in light 28Si acidity (protons and complexing organic acids) in temperate forests and in new tertiary minerals (tertiary kaolinite) in B horizons can only be the subsequent Podzol formation imply heavy 30Si enrichment in pedo- related to a Si source progressively enriched in light 28Si over time. genic clay minerals of E horizons relative to the “unweathered” clay 30 Kaolinite seems to play a key role in the successive formation of clay minerals in BC horizon; Δ SiE–BC increasing from +1.01 to +2.04‰ minerals as the content of 2:1 clay minerals is quite constant during pe- in ca. 200 years (Table 2). The preferential loss of light 28Si in weathered dogenesis in the soil chronosequence (Fig. 3). clay minerals in E horizons compared to the “unweathered” clay min- The preferential release and incorporation of light 28Si during disso- erals in BC horizon is recorded in the Si isotope signature of pedogenic lution and re-precipitation of clay minerals in the pedogenic Si pool ac- clay minerals on very short time-scale. Moreover, the Si isotope frac- count for the Si isotopic depletion/enrichment in the clay fraction over tionation between the “unweathered” clay minerals in BC and pedogen- 30 time in the podzolic chronosequence. The preferential incorporation of ic clay minerals precipitating in Bhs (Δ SiBC–Bhs)of−0.91‰ highlights light 28Si during precipitation of Si released from the dissolution of ped- that a part of light 28Si released in topsoil is used for re-precipitation in ogenic clay minerals (in E and Bh horizons) explains the increasing en- the subsoil (Table 2). As a consequence, Si isotope signatures in the clay richment in light 28Si in newly-formed clay minerals (tertiary clay fraction of soils should be tested in other systems to trace the modifica- minerals in Bhs horizon) during podzolization. This is confirmed by tions of pedogenic clay minerals insoil–plant systems, such as devel- the fact that pedogenic clay minerals in E horizons are increasingly oped in highly weathered tropical and subtropical environments heavier over time (Table 2), showing that the dissolution of pedogenic (Ferralsols, Lixisols, Nitisols, …), in frozen soils (Cryosols), in soils 28 J.-T. Cornelis et al. / Geoderma 235–236 (2014) 19–29

Fig. 6. Conceptual representation of the contribution of Si released from the dissolution of primary and secondary Si pools (lithogenic, biogenic and pedogenic) to the re-precipitation of new “tertiary” clay minerals during podzolization. Phase I → phase II (C → BC) = transition from the parent C material to the pedogenic BC horizon with neoformation of secondary clay minerals. Phase II = formation of typical podzolic soil horizons: E, Bh, Bhs, Bs and Bw. Phase II → phase III = transition from young to older Podzol characterized by (i) the deepening of 30 28 the E horizon where secondary clay minerals are weathered and enriched in heavy Si (EII → EIII), and (ii) B horizons tertiary clay minerals re-precipitate and are enriched in light Si (BCII

→ BhsIII). characterized by illuviation of clay minerals (Luvisols), in young soils newly-formed clay minerals (tertiary, quaternary …)comparedtothe (Cambisols) and in soils with high biological activity (Chernozems). Si contribution of lithogenic and biogenic Si pools. Our dataset shows isotope composition of pedogenic clay minerals can be useful to trace that the Si isotope compositions of soils are influenced not only by bio- and quantify the impact of environmental changes (temperature, rain- genic (phytolith formation/dissolution) and litho-, pedo-genic process- fall, acid deposition, land use …) on pedogenic clay evolution. This is es (primary mineral dissolution and secondary mineral precipitation) central to a better understanding of soil development and associated but also by a more advanced weathering process, i.e. successive forma- terrestrial biogeochemical processes. tion of pedogenic clay minerals. This should be taken into account when δ30Si values of the bulk soil and soil solutions are used for studying soil 5. Conclusions weathering degree and tracing dissolved and particulate Si transferred from soil–plant systems to the hydrosphere. The process of dissolution of pedogenic clay minerals during podzol- fi ization is con rmed by the Si isotopic signature of the clay fraction in a Acknowledgments podzolic soil chronosequence (Cox Bay, Vancouver Island). Our dataset shows Si isotopic, geochemical and mineralogical trends with depth and We thank A. Iserentant, C. Givron, P. Populaire, A. Lannoye, I. Caignet, as a function of pedogenic time, providing an orthogonal dataset which P. Sonnet, M. Detienne (UCL), H. Schreier, S. Smukler, B. Kieffer (UBC), sheds light on the origin and evolution of pedogenic clay minerals in the fi 28 as well F. Talbot and A. Cornelis for eld and laboratory assistance, clay fraction. The depletion in light Si in pedogenic clay minerals in V. Lai and M. Soon (UBC) for assistance in element analysis and topsoil increases over time (from +1.01 to +2.04‰)andapartof 28 28 K. Gordon (UBC) for assistance in Si isotopic analysis. We thank light Si released accounts for the relative enrichment in light Si in M. Brzezinski (University of California Santa Barbara) for providing us − ‰ pedogenic clay minerals in subsoil ( 0.91 ). This highlights that Si re- diatomite. J-T.C. is supported by “Fonds National de la Recherche leased from the partial dissolution of secondary clay minerals in topsoil Scientifique” of Belgium (FNRS; Postdoctoral Researcher Grant). This re- contributes to the neoformation of tertiary clay minerals in subsoil. Clay search was also supported by the “Fonds Spécial de Recherche” of the mineral dissolution has often been regarded as an irreversible process, UCL and by D.W. NSERC Discovery Grant. while the increase of 28Si enrichment over time in the clay fraction documented in this study indicates successive formation of clay minerals, which depends on the downward movement of the weathering front References in the soil. The continuous weathering of pedogenic clay minerals is Banwart, S., 2011. Save our soils. Nature 474, 151–152. an important process in the formation of Podzols as we show that the Calvaruso, C., Mareschal, L., Turpault, M.-P., 2009. Rapid clay weathering in the Si released in soil solution contributes to the reformation of clay min- rhizosphere of Norway Spruce and oak in an acid forest ecosystem. Soil Sci. Soc. Am. J. 73, 331–338. erals deeper in soils over very short time scales (ca. 300 years). The re- Caner, L., Joussein, E., Salvador-Blanes, S., Hubert, F., Schlicht, J.-F., Duigou, N., 2010. 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