Elemental composition of calcites in late Quaternary pedogenic calcretes from Gujarat, western India

Journal of Asian Earth Sciences 25 (2005) 893–902 www.elsevier.com/locate/jaes

Elemental composition of calcites in late Quaternary pedogenic calcretes from Gujarat, western India Aniruddha S. Khadkikar Agharkar Research Institute, G.G. Agarkar Road, Pune 411 004, India Received 21 November 2003; revised 21 June 2004; accepted 3 September 2004

Abstract Pedogenic calcretes commonly exhibit clotted micrite, circum-granular calcite (grain coats) and microspar/spar veins. The three calcitetypes with different dimensions were analyzed for their magnesium content to determine the relationship between crystal elongation and magnesium incorporation. The results show a very low MgO content for grain coats and microspars and high values for clotted micrite indicating that the ideal kinetic model does not hold true and several variables govern the end composition of calcites. The magnesium concentrations of meteoric calcites are genetically linked to the evolutionary history of the soil and climate. Grain coats, which are elongated calcites, contain the least amount of Mg and is related to the initial stages of pedogenesis wherein the limiting factor is the Mg/Ca ratio of the parent fluid. Lower magnesium contents arise due to smaller quantities of Mg being released during incipient weathering. Micrite morphology and composition is controlled by the greater availability of Mg ions through weathering, higher pCO2 in soil due to increased time-dependent soil respiration, which causes a rise in calcite precipitation rates and clay authigenesis. This in turn exerts a physical control on morphology by occluding pore space and providing numerous nuclei for calcite precipitation. The wide variability in spar cements is inherently controlled by inhomogeneties in parent fluid compositions with lower-than-micrite values on account of slower precipitation rates. q 2004 Elsevier Ltd. All rights reserved. Keywords: Calcrete; Caliche; Geochemistry; Meteoric; Calcite; Magnesium; Gujarat; India

1. Introduction Calcrete is a common feature of arid, semi-arid and subhumid landscapes wherein it forms either within the soil profile or through evaporative precipitation from groundwater in the phreatic and capillary-fringe zone (Goudie, 1983; Wright and Tucker, 1991; Tandon and Gibling, 1997; Khadkikar et al., 1998, 2000). It is also referred to as caliche, nari, kunkar, etc. (Goudie, 1983). Calcretes display characteristic fabrics and crystal morphologies in thinsection that aid in their discrimination (Wright and Tucker, 1991). Of these various microfabrics, the most commonly occurring are clotted micrite, circum-granular calcite (grain coats) and microsparitic/sparitic veins. These fabrics contain calcites having different dimensions. The calcite usually is low-Mg, although dolocretes are also known to E-mail address: [email protected] 1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2004.09.006

occur wherein the carbonate phase is dominantly, if not completely, represented by dolomite. Little is known about the elemental chemical composition of the carbonate phase of the calcretes. Electron microprobe analysis was carried out by Driese and Mora (1993) on Devonian calcretes from the central Appalachians. They analyzed spar cements and clotted micrite for Mg, Mn and Fe along three traverses, one containing a Microcodium spherule, another a micritic rhizolith and the third in a micritic nodule. These analyses yielded Mg concentrations varying from 2149 to 4244 ppm while spar cements had values from 562 to 4389 ppm. Hay and Wiggins (1980) reported microprobe analyses on pellets and ooids in calcrete profiles from California. The range in MgCO3 contents in spars was 2.5–5 mol% whereas the pellets and ooids had values between 1.5 and 0.1 mol%. Pliocene to Holocene calcretes from northern Tanzania contained up to 1 mol% MgCO3 (Hay and Redder, 1978).

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Herein results of electron microprobe studies on the carbonate phase in calcretes are presented and compared with bulk geochemical analyses on calcrete nodules. Moreover an attempt is made to understand the controls on the composition of meteoric calcites in these calcretes.

2. Quaternary geology The Quaternary sediments in Mainland Gujarat (Fig. 1) provide an excellent repository of information on changing palaeoenvironments over the past 130 kyrs. These sediments have formed under three discrete depositional environments representing an incremental weakening of the Southwest Indian Monsoon (Khadkikar et al., 1999). The stratigraphically lowest deposits (Figs. 1 and 2) represent ancient seasonal rivers that formed under a subhumid climate and are represented by a higher proportion of conglomerates and calcic Vertisols (Khadkikar et al., 1998, 1999). Stable isotope analyses on calcretes from the Vertisols have shown a dominance of a C3 dominated biomass (Khadkikar et al., 1999). These deposits have been dated using Electron Spin Resonance and luminescence based techniques which given an age bracket between 130 and 80 ka BP (Khadkikar et al., 1999). Upwards through the succession, the deposits become sand dominated and contain sediments deposited by ephemeral rivers (Khadkikar et al., 1999). A prominent ferric-calcisol (sensu Mack

et al., 1993) is seen throughout the area (Fig. 2), which documents climatic amelioration between 50 and 20 ka BP (Khadkikar et al., 1999). This phase of sedimentation continued till about 20 ka BP after which it is succeeded by ubiquitous sandy loess deposits; relicts of intense dust storm activity in tandem with the Last Glacial Stage (Khadkikar et al., 1999). The calcretes sampled for the present study come from the older two phases of sedimentation, i.e. deposits of seasonal and ephemeral rivers. Their detailed morphology has been described by Khadkikar et al. (1998), who gave a broad framework of calcrete production and recycling in semi-arid alluvial systems. Samples for microprobe studies were taken from the basal calcic Vertisol at Mahudi and Rayka and from the ferric-calcisol at Dabka. These calcrete samples were preferentially chosen due to their enrichment of calcite and minimal siliciclastic content. 3. Methodology Based on the enrichment of calcium carbonate seen petrographically, three specimens, two of pedogenic calcrete nodules from calcic Vertisols and one of a pedogenic calcrete nodule from a ferric-calcisol, were chosen for electron microprobe analyses. Analyses were carried out on a JEOL 733 electron microprobe equipped with four wavelength dispersive spectrometers and an Oxford Link eXL energy dispersive system at the Department of Earth Sciences,

Fig. 1. Location map of the sites from which the calcrete samples were taken.

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Fig. 2. Graphic logs of the sites showing the various soil and calcrete types, occurring throughout the section. Also shown are the levels from which the calcretes for bulk-geochemistry and EPMA analysis were taken.

Dalhousie University, Canada. The energy dispersive system was used for all elements. The resolution of the energy dispersive detector was 137 eV at 5.9 keV. Each spectrum was acquired for 40 s with an accelerating voltage of 15 kV and a beam current of 15 nA. The raw data were corrected using Link’s ZAF matrix correlation program. The accuracy for major elements was G1.5–2.0%. Detection limits for most elements ranged from approximately 0.1 to 0.3%. However, it should be noted that due to the coarser finish of the thin sections, the actual errors might be up to 10%. The XRF analyses were carried out using a Siemens SRS 3000 Sequential X-ray spectrometer with END WINDOW Rh X-ray Tube at the Wadia Institute of Himalayan Geology, Dehra Dun, India on pressed powder pellets. The overall accuracy (%RSD) for all major and minor oxides was !5% and for trace elements !12%.

calcite changes into bladed sparite. In all cases there is only one stage of crystal growth, i.e. it is singly tiered. Rare equant sparite rim cements are also observed. Sinuous veins of calcite cut randomly across in all thin sections except from those calcretes from the ferric-calcisol at Dabka. These veins are filled with sparite or microsparite and are directly linked with the width of the vein which ranges between 16 and 72 mm. The drusy spar changes in its dimension from the walls where it is 8–12 mm to the center of the vein where it is up to 50 mm in size. Sparitic veins occur in isolation or may contain offshoots. All veins narrow at their terminations, which are usually directed towards the interior of the nodule. Sometimes the growth vectors of calcites are directed inwards from the clotted micrite wall but do not fill the vein, leaving some vacant space between oppositely facing crystal face terminations. Calcite veins also contain floating micritic intraclasts and quartz grains, but such instances are few.

4. Petrography Clotted micrite, occurring as dense accumulations of microcrystalline calcite !4 mm in diameter are observed in most thin sections. The micrite shows varying degrees of transmittance and is usually light brown in color. At places microsparitic growth is common and these occur as irregularly distributed patches. Clotted micrite in most samples form the bulk of the section. Most non-carbonate grains are enveloped by a coat of fibrous calcite. This coat is often of equal thickness, but in some cases it is thicker on one side of the grain. The calcites are usually needle-like in form but show bladed forms also. The coats are up to 8 mm thick irrespective of grain composition and shape. The coats surround clasts of quartz, feldspar, mica and tourmaline also. In places needle-like

5. Geochemistry 5.1. Palaeosol geochemistry The Vertisols at Rayka and Mahudi and the ferric-calcisol at Dabka were sampled for geochemical analyses in order to gain insights on the chemical environments that existed during the formation of the calcretes. The macromorphology of the palaeosols have been described in detail elsewhere and shall not be repeated here (Khadkikar et al., 2000). The chemical data (Table 1) as well as ratios were plotted to gain insights on the nature and extent of weathering and soil chemical environments (Fig. 3). The plot of TiO2 against the ratio Al2O 3/Bases shows overlapping fields between

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Table 1 Geochemical data for the three palaeosols Clotted micrite CaO (%)

Spar in veins MgO (%)

Hydromorphic vertisol calcrete (Mahudi) 53.0280 1.0190 51.8590 0.7820

Vertisol calcrete (Rayka) 48.0110 0.8660 47.8080 1.2370

Grain coats

CaO (%)

MgO (%)

CaO (%)

MgO (%)

54.5220 54.9220 54.0100 52.8490 54.4630 56.2360

1.0030 0.1950 0.5110 0.8200 1.1260 0.9790

53.8930 53.7660 55.2900

0.5340 0.4710 0.2750

52.9490 53.8920 53.1260 53.8950 54.3780

0.8230 0.9790 0.1820 0.7790 0.7900

53.6500 53.1150

0.7910 0.5740

52.2170 53.0780 53.2720 53.4950

0.2270 0.5270 0.3570 0.2920

Ferric-calcisol calcrete (Dabka) 46.9060 1.0140 49.3090 0.9790 50.0750 1.4230 50.8820 0.6040 51.6210 0.3750

the Dabka ferric-calcisol and Rayka Vertisol, while the Mahudi Vertisol samples plot in the extreme right (Fig. 3). A positive correlation is observed between these variables. High values of MnO are seen for the Rayka and Dabka soils while the Mahudi Vertisol has the lowest values, but a very high iron content of the three. MgO and CaO on the other hand is depleted in the Rayka and Dabka paleosols and enriched in the Mahudi paleosol by a factor of two. The alumina vs. silica plot best differentiates the three types of soils (Fig. 3). In the Rayka Vertisol, alumina varies between

10 and 12% and is similar to the Dabka paleosol, whereas the Mahudi Vertisol is relatively enriched. The Dabka paleosol on the other hand is enriched in silica with progressively lower values for the Mahudi and Rayka Vertisol, respectively. The Ba–Sr plot (Fig. 3) shows a constant range in Ba concentrations between 200 and 350 ppm for the three paleosol types, but differences arise in their Sr content. The Dabka paleosol has the lowest Sr content followed by the Mahudi Vertisol and the Rayka Vertisol, which has the highest values ranging between 200 and 240 ppm.

Fig. 3. Bivariate plots of various indices that reflect the physico-chemical environment of the soils (Retallack, 1990). For details see text.

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Table 2 XRF bulk geochemical data for representative calcretes from each palaeosol Sample number

Description

CaO (%)

MgO (%)

SiO2 (%)

Al2O3 (%)

M/V1

Nodule from Mahudi basal vertisol Nodule from Mahudi vertisol overlying MV1 Nodule from Mahudi vertisol overlying MV2 Nodule from Rayka vertisol Nodule from Dabka ferric-calcisol Dabka from Poicha ferric-calcisol

50.26

0.64

10.29

1.80

44.83

0.68

14.28

2.41

45.58

0.70

16.85

2.41

40.62

1.37

17.97

3.18

42.20

0.86

19.17

2.71

53.46

0.54

5.34

1.00

M/V2

M/V3

R/V3 D/RS P/RS

5.2. Calcrete bulk-geochemistry The bulk-geochemistry of calcretes provides the total composition of the nodule which includes both carbonate and non-carbonate components. A total of six samples were obtained, of which four were of calcrete nodules from Vertisols and two were of nodules from ferric-calcisols. CaO values for all calcretes ranged between 40 and 50% (Table 2). A far wider range in MgO was observed for Vertisol calcretes (0.6–1.4%) than for ferric-calcisol calcretes (0.5–0.8). Silica contents ranged between 5 and 18% (Table 2) whereas alumina varied between 1 and 3%.

No values were found typical for either nodule types. A plot of SiO2CAl2O3 (representative of non-carbonate constituents) and CaOCMgO (representative of carbonate constituents) shows that the Gujarat samples plot around the global average composition of calcrete (Goudie, 1983). 5.3. EPMA analyses of calcites in calcretes Distinct zones in bivariate plots of CaO vs. MgO are observed for spar/microspar veins, clotted micrite, and grain coats (Fig. 4; Table 3). In spars, MgO varies from 0.1 to 1.25% but has a restricted range of CaO between 53 and 56%.

Fig. 4. Bivariate plots of MgO and CaO (values in weight%) for spar, grain coats and micrite for the three varieties of calcretes. This is to examine the variations in composition in individual fabric types.

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Table 3 Electron microprobe data for individual microfabric types from each variety of calcretes nodule Sample

SiO2 (%)

Al2O3 (%)

CaO (%)

MgO (%)

Na2O (%)

K2O (%)

MnO (%)

Fe2O3(T) (%)

TiO2 (%)

Ba (ppm)

Sr (ppm)

Mahudi vertisol

58.75 64.25 68.96 67.95 49.80 54.34 47.41 53.59 73.09 71.60 69.83 71.62

18.75 14.06 13.39 13.72 11.62 11.74 10.51 11.73 9.35 10.21 10.48 9.42

0.80 1.15 0.65 0.62 9.59 6.94 11.81 6.17 2.73 2.37 1.40 2.54

2.24 2.37 1.84 1.88 3.74 3.82 3.65 4.29 1.68 1.80 1.88 1.86

0.57 0.59 0.62 0.53 0.655 0.80 0.63 0.80 1.08 1.02 1.06 1.21

2.71 2.01 2.10 1.44 0.93 1.23 0.90 1.33 1.77 1.82 1.87 1.74

0.06 0.04 0.03 0.03 0.06 0.08 0.11 0.12 0.07 0.07 0.09 0.09

8.40 7.20 5.88 7.19 6.16 6.68 5.37 6.54 5.10 5.51 5.97 5.39

1.07 1.10 0.97 1.18 1.04 1.07 0.84 1.05 0.88 0.95 0.99 0.92

517 337 318 257 276 308 322 387 264 289 301 292

193 216 209 177 222 192 212 248 139 127 126 139

Rayka vertisol

Dabka ferric-calcisol

The mean value of MgO in spar is 0.8%. Sparitic veins are not present in the calcite from the Dabka ferric-calcisol. No segregation in MgO and CaO is observed for the two Vertisols. Micrite on the other hand shows different values for the three calcrete samples (Fig. 4; Table 1). Whereas the Rayka Vertisol calcrete has MgO ranging between 0.8 and 1.25% and corresponding CaO around 48%, micrite from the Mahudi calcrete has comparatively lower values. MgO lies around 0.7–1.1% and CaO values are between 52 and 54%. The largest variation is observed in the Dabka calcrete which shows a range in MgO from as high as 1.5% to as low as 0.4%. CaO also changes from 52 to 47%. However, there is no correlation between CaO and MgO values. Micrite also

showed trace amounts of alumina, which ranged between 0.25 and 0.6%. Grain coats show the smallest range in values for all three calcretes (Fig. 4; Table 1, plotting in the lower right corner. Within this ensemble, the Rayka calcrete have the highest MgO values and the Dabka calcrete the lowest. CaO concentrations are fairly uniform between 52 and 56%. The same data were re-examined for each calcrete type in order to understand intra-sample variations for the three different calcite morphologies (Fig. 5). In the Mahudi calcrete, micrite and spar have higher MgO concentrations compared to grain coats. The largest variation in both MgO and CaO contents are observed for spar (Fig. 5). For

Fig. 5. Bivariate plots of MgO and CaO (values in weight%) for spar, grain coats and micrite for the three varieties of calcretes. This is to examine the variations that have arisen due to differing physico-chemical conditions specific to each soil type on the composition of the microfabric.

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the Rayka calcrete, a similar range is seen in spar but with narrowly constricted CaO values around 54%. Micrites have larger concentrations of MgO (1–1.25%) relative to the grain coats (0.5–0.75%). The Dabka calcrete shows two distinct fields for micrite and grain coats, the latter being relatively enriched in CaO and the former exhibiting the widest range in MgO levels (Fig. 5).

6. Discussion The geochemistry of calcretes contains useful information that can be related to the physico-chemical environments of the soil in which they form. These external variables exert control on the morphology and composition of meteoric calcites in calcretes as discussed below. The results clearly indicate that there exists a succinct relationship between the calcite microfabric and the Mg content, albeit a more complicated one than usually thought based on experimental and theoretical considerations. The relationship between Mg in calcite and crystal elongation is quite complicated. The major factors that control Mg content in calcite are (Folk, 1974; Lahann, 1978; Mucci and Morse, 1983): 1. Crystal growth rate; 2. Composition of co-existing solution; 3. Temperature. Given and Wilkinson (1985) extended the kinetic model of Lahann (1978) in which precipitation along the c-axis direction is limited by the availability of CO2K 3 ions. Mg incorporation along the side faces in their model is not governed by the Mg/Ca ratio in the co-existent fluid but is controlled by precipitation kinetics. Due to the slower growth rates, magnesium may also be expelled (Given and Wilkinson, 1985) resulting in lower (net) concentrations in slowly precipitating systems. Assuming that the kinetic model of Given and Wilkinson (1985) is true and that there exists an one-to-one correspondence among calcite crystal elongation and Mg content, for the Gujarat calcretes grain coats (which are the most elongated crystals) would have the highest magnesium content of the three microfabrics. However, this is not the case for grain coat calcites from the Gujarat calcretes (Fig. 6) which show a limited range between 0.2 and 0.8% (Table 3). Usually the values are much lower than both micrite and sparitic veins. Micrite on the other hand shows the highest values of Mg, ranging between 0.4 and 1.4%, with most of the values close to 1.0%. The possibility of fine particulate Mg-bearing clays within the micrite may be thought to govern the larger Mg values in the Gujarat calcretes. Bulk X-ray diffractometry analyses on the three calcrete-nodules show the presence of calcite as the singular carbonate phase followed by some quartz. Moreover if at all there is some particulate clay as several microprobe results reveal, it is possible to estimate roughly the amount of

Fig. 6. Summary diagram showing the range in chemical compositions (based on electron probe data). The error bars include the range in composition as well as errors in measurements and the contribution of Mgclays. The arrow shows the general temporal path observed in thin sections (grain coats followed by micricte followed by spar) of the calcretes.

MgO that may be associated with the clays minerals. Bulk geochemical data on the paleosol clays (Table 1) give an MgO/Al2O3 ratio ranging between 0.12 and 0.36 with a mean value of 0.22. If one assumes that the total alumina observed in the calcrete is derived from the Mg-bearing clays, then the above ratio may be used to determine the amount of Mg associated with the alumina concentrations. Hence, for alumina levels of 0.25–0.6%, the corresponding MgO concentrations would range between 0.05 and 0.1% (estimated using the equation ðMgOcalcite Z ½MgO=Al2 O3 paleosol !Al2 O3ðcalciteÞ Þ: This implies that the 10% maximum error assumed for Mg concentrations is compensated by the contribution of Mg from the interstitial clays. This in turn does not affect the following arguments. Under the kinetic model, the rate of crystal growth determines the Mg content of calcites; hence longer calcites mean more Mg is incorporated. Thus, in the calcretes studied from Gujarat, micrite would be expected to have lower Mg concentrations unless governed by other variables. Finally, the last microfabric in the Gujarat calcretes, vein-filling spar, has lower concentrations than micrite but contain more Mg relative to grain coats. This again is in contrast with the ideal kinetic model. This suggests that in natural continental environments, numerous variables determine the final concentrations of Mg in meteoric calcites. These incongruencies are readily explained when examined in the broader context of the evolutionary history of the soil. Let us consider a fresh land surface on which pedogenesis initiates in an alluvial plain. During these initial stages, the composition (Ca/Mg) of meteoric waters is primarily governed by aerosols (Machette, 1985) and to some extent by incipient weathering in the solum (Fig. 7). Weathering plays an important role in releasing Mg ions through the weathering of mafic minerals in basalt. Thus, in the initial stages of the soil, owing to

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Fig. 7. Model summarizing the various parameters that have controlled the end compositions of the meteoric calcites with the relative contribution of each environmental parameter.

incipient weathering, the availability of magnesium would be considerably low. This lower availability of Mg causes the grain coat cements in the Gujarat calcretes (Mg content between 0.2 and 0.8%) to be depleted in Mg, the limiting factor being the Mg/Ca ratio of the ambient fluid. The next dominant form of calcite in the calcretes studied is micrite. Micrite in the calcretes from Gujarat show higher concentrations of Mg as compared to the earlier precipitated grain coats (w1.0%). As weathering proceeds, the decomposition of primary silicate releases more Mg in the solum, some of which is arrested through the formation of Mg-smectites. All the palaeosols from which the calcretes were sampled show the presence of smectitic clays. Such clays also serve to occlude pore space and reduce the size of the pores themselves. This reduction in pore dimensions exerts a physical control on the size of the precipitating calcite. Wieder and Yaalon (1974) also concluded, based on their study of the northern Negev (Israel), that soil matrix has a considerable influence on calcite size such that dispersed clays serve as nucleation sites and subsequently also act as inhibitors. Another major control on calcite precipitation kinetics is the partial pressure of carbon dioxide in the soil, which is governed by the organic productivity of the soil. Typical values of pCO2 of the atmosphere are around 0.035 kPa, which increases considerably in the solum (10–100 times) due to soil respiration and transport (Mcfadden et al., 1991; Chadwick et al., 1994). Lebron and Suarez (1998) recently reported that at constant supersaturation, the precipitation rate increased in response to higher pCO2 levels. They also found a linear relationship between the activity of CaHCOC 3

and calcite precipitation rate under a range of pCO2 values from 0.035 to 10 kPa, suggesting that higher precipitation rates are related to the increase in the negative charge on the calcite crystal surface due to the increase in the activity of CaHCOC 3 . The combined influence of higher Mg/Ca ratio of parent fluid, higher soil pCO2 levels due to time-dependent rise in organic productivity of the soil and the physical influence of smectitic clays results in the higher Mg values (w1.0%) in micrite in the Gujarat calcretes (Fig. 7). With progressive weathering, the amount of weatherable minerals decreases, resulting in the decline in the amount of Mg released through this process (Fig. 7). Higher contents of smectite lead to the predominance of shrink–swell processes in the soil (Wilding and Tessier, 1988), which also affect the carbonate nodules (Khadkikar et al., 1998, 2000). These shrink–swell processes cause cracking of the nodule surface within which percolating meteoric waters depleted in Mg can reside. This sheltered residence leads to the precipitation of spar and microspar under very slow rates. Such slow rates and the possibility of dissolution of the surrounding micrite leads to a wide range in Mg content in vein calcites from the calcretes of Gujarat, although lower than micrite. Mg concentrations in grain coats from Mahudi and Rayka Vertisol calcretes show relatively higher values for the former. As grain coats formed during the initial stages of soil formation, the soil geochemical data may not altogether represent the soil environment that persisted then. Either it may be assumed that both paleosols began developing from identical physico-chemical states or, based on

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the geographical distance from the principal channel tract, began forming on substrates containing different proportions of detrital clay minerals. Hence the higher values for the Mahudi calcrete grain-coats could well represent differences in the composition of the parent fluids. Significant variations are also seen from paleosol to paleosol for the three types studied. The paleosols themselves reflect different stages and types of weathering which are reflected in their bulk-geochemistry. A higher degree of weathering is evident in the silica– alumina ratio for the Mahudi Vertisol, which is enriched in alumina. In comparison, the Dabka paleosol is more enriched in silica accompanied by lesser amount of alumina, indicating a higher proportion of quartz and unweathered feldspathic minerals. The Rayka Vertisol shows moderate weathering in its low alumina and silica contents. Base leaching is at a maximum for the Mahudi Vertisol whereas the remaining two paleosols show similar degrees of leaching. Contrasting soil drainages are manifested in the MnO–Fe2O3(T) plot with the Mahudi paleosol showing poor-drainage due to the depletion of MnO (Retallack, 1990). The Ba–Sr plot shows similar values for the two Vertisols but is depleted in Sr for the Dabka paleosol. Mahudi calcretes show more enrichment in CaO and lower MgO values than the Rayka calcretes, which may be due to the increased sequestration of Mg in smectites leading to lower Mg/Ca ratios in the parent fluids. The higher CaO content agrees with the paleosol chemistry data, which shows more leaching of bases from the non-carbonate mineral phases and impeded drainage conditions. The lower CaO values for the other two calcretes may be related to lower baseleaching and free drainage as in the case of the Dabka palaeosols and is also reflected in the TiO2/(Al2O3/ CaOCMgOCNa2OCK2O) plot (Retallack, 1990). Whereas the Rayka and Mahudi Vertisols are stratigraphically equivalent (Khadkikar et al., 1999) and formed under identical climates, the Dabka paleosol differs in environmental conditions under which it developed and is stratigraphically higher (younger). Climates during the formation of Vertisols have been interpreted by Khadkikar et al. (1999) to be sub-humid, whereas a semi-arid climate prevailed during the formation of the Dabka ferric-calcisol. These differences in climate imply different vegetation cover and density, mean annual rainfalls and soil temperatures, which may have controlled the differences in the Mg contents of the calcites between the two types of pedogenic calcretes. The Dabka calcrete grain-coats have significantly lower concentrations of Mg as compared to the Vertisol calcretes. This may be due to dissimilarities in climate and vegetation, which was more arid and C4 biomass dominated (Khadkikar et al., 2000) during the formation of the Dabka paleosol and C3 biomass dominated and sub-humid during the formation of the Rayka and Mahudi palaeosols. This may eventually have affected

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the chemistry of meteoric water and its ability to decompose minerals marginally. These different types of climates would have resulted in different soil pCO2 levels through changes in the vegetation biomass, lower soil temperatures and higher mean annual precipitation during the formation of Vertisols. These insights suggest that the controls on meteoric calcite composition occur at two scales. At the regional scale, climatic change causes changes in the vegetation biomass and rainfall and temperature, which in turn affects rates of weathering and soil drainage conditions and may be collectively grouped under ‘allogenic processes’. At the scale of the soil, such regional changes result in different states of soil temperature, soil pCO2 and rates of weathering (biogenic and inorganic). The three principal forms of calcites seen in the calcretes, form at various stages in the evolutionary history of a soil with some overlap. The chemical composition of these calcite microfabrics (clotted micrite, grain coats and sparitic/microsparitic veins) are intricately related to changes in Mg/Ca ratio of parent fluid, time-dependent clay content increase in the soil and its effect on the physical properties of the soil and may be lumped together as ‘autogenic processes’.

7. Conclusions The magnesium concentrations of meteoric calcites are genetically linked to the evolutionary history of the soil (autogenic variable) and climate (allogenic variable). There is no simple correlation between crystal morphology and composition as numerous variables operate simultaneously to control the outcome. Grain coats, which are elongated calcites, contain the least amount of Mg and are related to the initial stages of pedogenesis wherein the limiting factor is the Mg/Ca ratio of the parent fluid. Lower magnesium contents arise due to smaller quantities of Mg being released during incipient weathering. Micrite morphology and composition is controlled by higher availability of Mg ions through weathering and higher pCO2 in soil due to increased time-dependent soil respiration. The latter causes a rise in calcite precipitation rates and clay authigenesis which exerts a physical control on morphology by occluding pore space and providing numerous nuclei for calcite precipitation. The wide variability in spar cements is inherently controlled by inhomogeneties in parent fluid compositions with lower-than-micrite values on account of slower precipitation rates. Differences in Mg abundances in the calcic Vertisol and ferric-calcisol are explained through dissimilar climates between 130–80 and 50–20 ka BP, which influenced (a) soil pCO2 through changes in the vegetation biomass, (b) temperature and (c) mean annual precipitation.

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