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Isotope geochemistry of caliche developed on basalt
Geochimica et Cosmochimica Acta, Vol. 67, No. 2, pp. 185–195, 2003 Copyright © 2003 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/03 $22.00 ⫹ .00
Pergamon
PII S0016-7037(02)01051-7
Isotope Geochemistry of Caliche Developed on Basalt L. PAUL KNAUTH,*,1 MAURO BRILLI,2 and STAN KLONOWSKI1 1 2
Department of Geological Science, Box 85287-1404, Arizona State University, Tempe AZ85287-1404 USA Department of Geological Sciences University of Roma “La Sapienza”P.le A. Moro, 5 - 00185 Roma (Italy) (Received January 30, 2002; accepted in revised form July 15, 2002)
Abstract—Enormous variations in oxygen and carbon isotopes occur in caliche developed on ⬍ 3 Ma basalts in 3 volcanic fields in Arizona, significantly extending the range of ␦18O and ␦13C observed in terrestrial caliche. Within each volcanic field, ␦18O is broadly co-variant with ␦13C and increases as ␦13C increases. The most 18O and 13C enriched samples are for subaerial calcite developed on pinnacles, knobs, and flow lobes that protrude above tephra and soil. The most 18O and 13C depleted samples are for pedogenic carbonate developed in soil atmospheres. The pedogenic caliche has ␦18O fixed by normal precipitation in local meteoric waters at ambient temperatures and has low ␦13C characteristic of microbial soil CO2. Subaerial caliche has formed from 18O-rich evapoconcentrated meteoric waters that dried out on surfaces after local rains. The associated 13C enrichment is due either to removal of 12C by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation. Caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over-layered during burial by calcite carrying the isotopic signature of the soil environment. The large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if Mars had a “warm, wet” early period and photosynthesizing microbes were present in the early soils. The approach can be similarly applied to terrestrial Precambrian paleocaliche in the search for the earliest record of life on land. Large variations reported for ␦18O of carbonate in Martian meteorite ALH84001 do not necessarily require high temperatures, playa lakes, or flood runoff if the carbonate is an example of altered martian caliche. Copyright © 2003 Elsevier Science Ltd Aside from general interest in caliche on basalt as a terrestrial weathering phenomenon, a better understanding of its isotope systematics could provide a relatively simple strategy for assessing whether aqueous weathering ever occurred on Mars. If Mars had a “warm, wet” early period (e.g., Baker, 2001), then carbonate would have been produced during weathering via the Urey reaction. This weathering product could remain present today as caliche on and within some of the ubiquitous basalt boulder fields (whether retained in situ at the site of weathering or formed from the wind-blown weathering products of basalts in more distant areas). Wind abrasion at rates as high as 0.21 mm/yr (Greeley et al., 1982) has likely removed any exposed caliche on surfaces, but this white material may currently exist in protected cracks, internal vesicles, and under rocks where it could be readily recognized and sampled. On Earth, the oxygen isotopes are fixed at the time of caliche precipitation and are governed by the temperature and isotopic composition of the local meteoric waters. The C isotope composition is determined by the ratio of atmospheric CO2 to soil CO2 produced by microbes (Cerling, 1984). Caliche forms on recent basalt-flow surfaces that are populated by an abundance of lichens, plants, mosses, and microorganisms. It visibly encrusts roots and almost certainly entombs pollens and microbes. The possibility that microbial life was present on Mars during the suggested “warm, wet early” period is currently being seriously explored (e.g., Farmer, 1998). Because of its intimate association with microbial life, any caliche on Mars could well have preserved isotopic (if not microfossil) evidence of any such past life. The isotope geochemistry of carbonate developed in soils
1. INTRODUCTION
In arid regions on Earth, masses of microcrystalline carbonate known as “caliche” are found in vesicles, along fractures, and/or coating surfaces in young basalt flows and cinder cones (Fig. 1). It can become thickly and extensively developed throughout the basalt in soils and in the shallow subsurface. Caliche is common in arid-region soils developed on rocks of all types where it apparently originates mostly by dissolution and reprecipitation of wind-blown carbonate dust (Brown, 1956; Pe´we´ et al., 1981; Capo and Chadwick, 1999). Sr isotope studies show caliche found on weathered basalt contains a mixture of Ca derived from the basalt and Ca derived from wind-blown dust (Naiman et al., 2000). The amount derived from the basalt ranges from 100% for examples in Hawaii (Capo et al., 2000) to less than 50% in areas where surrounding limestones, playas, or sea spray contribute large amounts of wind-blown material (Quade et al., 1995; Naiman et al., 2000). All carbonate on Earth is ultimately derived from the weathering of silicate rocks via reactions generalized in terms of the famous “Urey Reaction” (Urey, 1952): CO2 ⫹ CaSiO3 ⫽ CaCO3 ⫹ SiO2 For basalt, CaSiO3 is a proxy for Ca, Mg silicates such as plagioclase and pyroxene. Whether derived in situ or by recycling of wind-blown carbonate, the oxygen and carbon isotopic composition of caliche has repeatedly been shown to be completely reset during weathering to values determined by the local conditions of weathering (e.g., Cerling and Quade, 1993). * Author to whom ([email protected]).
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isotope variations in terrestrial caliche and argue that a strong isotopic biosignature is present in caliche that develops on basalt. The results carry implications for astrobiological prospecting on Mars and also apply to the search for life on land in the Precambrian and to interpretations of stable isotope variations in carbonate in the martian meteorite ALH 84001. 2. SAMPLES, PROCEDURES, AND NOTATION
Fig. 1. A. White caliche coating a vug exposed on the side of a pinnacle of basalt on the 900 yr– old Kannanna Basalt Flow, Sunset Crater, Arizona. Botyroidal microcrystalline calcite is most thickly developed along the top of the cavity. Pocketknife is 6 cm. Locality for sample #755. B. White caliche coating fracture surfaces on older (1 to 3Ma) flow in the San Francisco Volcanic Field, Arizona. A large block was removed for the road cut and exposed caliche– coated fracture surfaces. Hammer is 36 cm. Locality for sample #686. C. Road cut exposure of pedogenic caliche (sample #742) developed in heavily weathered basalt. Hammer handle length is 36 cm. Locality for sample #740.
has been extensively studied (see review by Mermut et al., 2000), but caliche developed specifically on young basalt flows has received very little attention. Consequently, we investigated samples from 3 young volcanic fields in Arizona that contain basaltic lavas ranging in age from 900 yr to ⬃3 Ma. We report data that significantly extend the range of C and O
Field examination revealed two basic types of caliche: 1) Micro-layered and botyroidal accumulations in open spaces such as fractures, amygdules, and cavities developed on cinder cones and flow-deformed mounds of solidified lava; and 2) classic pedogenic accumulations in clay-rich soils developed on the older, more weathered flows. The pedogenic caliche includes indurated, layered horizons and cross-cutting veins of calcite and calcite-cemented basalt rubble. For the purposes of this study, the important distinction is between calcite that has formed in a soil atmosphere (“pedogenic”) versus that which has precipitated in open spaces on pinnacles and lobes of relatively unweathered flows and cones which stand above the local tephra and/or soil (“subaerial”). No attempt was made in this initial survey to define soil types and sample systematically within soil horizons. The samples include the complete spectrum of caliche readily observed in 3 separate volcanic fields. Global Positioning System coordinates of all samples are given in (Electronic Supplement, ES3277, www.elsevier.com) Table 1. 10mg to 20 mg samples were quarried, drilled-out, or scraped from hand samples and analyzed with the original off-line green phosphoric acid technique of McCrea (1950). CO2 so liberated was analyzed on a 15 cm Nier isotope ratio mass spectrometer and corrected for instrumental effects (Craig, 1957). XRD analysis of several typical samples from the older flows confirmed that the pedogenic carbonate was exclusively calcite. Mg carbonates and even dolomite have developed by the direct weathering of young basalts in Hawaii (Capo et al., 2000; Whipkey, 2002), so it is possible that some of our “subaerial” carbonates on the younger flows have a more complicated mineralogy. Oxalates and sulfates are also possible, but none were detected in the limited number of XRD analyses. The phosphoric acid correction of Sharma and Clayton (1965) for calcite was used so that the standard ␦18O notation here refers to lattice oxygen. ␦18O and ␦13C were referenced against lab standards calibrated against the original PDB standard. ␦-values obtained in this fashion are within experimental error (⫾0.2‰ for O and ⫾0.1‰ for C) of those referenced to VPDB. ␦18O values were converted to the SMOW scale using 1.0412 for the CO2-H2O fractionation (O’Neil et al., 1975). All data are given in Table 1. 3. SAN FRANCISCO VOLCANIC FIELD, ARIZONA
The 900 yr-old Kananna basaltic lava flow emerges to the northeast from Sunset Crater (22 km northeast of Flagstaff, Arizona) and lies on a stack of successively older flows as old as 3 Ma (Moore and Wolfe, 1987). Subaerial caliche on the very young Kananna flow is sparsely developed and requires considerable searching to locate. However, it locally reaches thicknesses up to 1 cm along fractures and especially on un-
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Fig. 2. Isotopic data for caliche in the San Francisco Volcanic Field near Flagstaff, Arizona. The data– envelopes are for various stratigraphic ages of the host lavas as mapped by Moore and Wolfe (1987). The most 13C and 18O enriched samples are from pinnacles of a 900 yr– old flow that have never been buried and exposed to a soil atmosphere. The most 13C and 18 O depleted samples occur in thickly developed pedogenic caliche.
derlying surfaces in spaces probably created during flow deformation of solidifying lava (Fig. 1A). It is thickly and widely developed within vesicles and along fractures in the older flows (Fig. 1B). Soils are extensively developed on the older flows throughout the volcanic field and these contain abundant beds, plates, and cross cutting veins of pedogenic caliche (Fig. 1C). Sampling focused on localities near the north end of the Kananna flow where lavas of multiple ages are exposed within a strip 2 km X 15 km (Moore and Wolfe, 1987). Other samples of pedogenic caliche on the older flows were obtained from areas several km to the southeast. Isotopic data for the San Francisco Volcanic Field fall into a broad, sloping array on a ␦13C -␦18O plot in which caliche developed on the youngest lavas is strongly enriched in 18O and 13 C relative to that developed on the older lavas (Fig. 2). The isotopic variations of 18‰ in both ␦18O and ␦13C are truly enormous; samples from this small geographic area display isotopic variations that rival or exceed the range of marine and terrestrial limestones and dolostones, all taken together. ␦13C values rise to ⫹10‰, whereas pedogenic carbonate worldwide almost never exceeds ␦13C values of ⫹5‰. ␦18O is as large as ⫹31.6‰, which is also higher than nearly all published values
for pedogenic carbonate in soils developed on parent rocks of all types. In terms of isotopic composition, this represents a new type of 13C and 18O enriched caliche not reported previously. The most 13C and 18O enriched samples are from the sparsely developed caliche developed on subaerial pinnacles and mounds of 900 yr old lava which is completely free of recognizable soil development and has never been buried and exposed to a soil atmosphere (Fig. 1A). The most 13C and 18O depleted samples are pedogenic caliche developed in the soils of the deeply weathered older flows (“Woodhouse” age; Fig. 1B and 1C). Samples from lavas with intermediate ages (Tappan, Merriam) generally have ␦-values intermediate with respect to oldest and youngest lavas. 3.1. Co-variation of ␦18O and ␦13C in Caliche Oxygen and hydrogen isotopic measurements of precipitation over a 13 yr period at Flagstaff, Arizona, within ⬃300m elevation and within 35 km of the sample areas reveal a mean ␦18O of ⫺8.1‰ (IAEA, 1981). Yearly means vary by over 15‰. The mean temperature during this interval was 7.5°C
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(IAEA, 1981). Calcite in isotopic equilibrium with water with ␦18O ⫽ ⫺8.1‰ at 7.5°C is ⫹24.6‰ (using the CaCO3-H2O fractionation of O’Neil et al., 1969). However, this area is covered with snow for long periods during the winter and it is likely that caliche formation proceeds preferentially during warmer times of the year. At 22°C, ␦18O of the caliche would be ⫹21‰, a value characteristic of the lower left end of the broad sloping array in Figure 2. Soil water is likely to have an oxygen isotopic composition that approximates the long term mean value for the local precipitation, so we interpret the lower left end of the array to represent caliche that formed in soil waters during the non-winter months at temperatures of ⬃7 to 22°C. Pedogenic samples with higher and lower ␦18O values have formed at higher or lower temperatures, respectively, and/or during periods when precipitation had higher or lower ␦18O values than average. Caliche developed within soils of the older flows has negative ␦13C-values ranging from ⬃0‰ to ⫺6‰. These are similar to other soil carbonates worldwide that have precipitated in the presence of respired CO2 modified by diffusive fractionation and mixing with atmospheric CO2 (Salomons et al., 1978; Cerling et al., 1991). At temperatures of 7 to 22°C, ␦13C of soil CO2 in isotopic equilibrium with such calcite of 0‰ to ⫺6‰ is approximately ⫺19‰ to ⫺11‰ using Bottinga (1968) and ⫺17‰ to ⫺9‰ using Romanik et al. (1992). The most negative calcite ␦13C values likely represent soil CO2 with the highest component of respired CO2. The value of ⫺19‰ is higher than expected for C3 plants which normally range from about ⫺20‰ to ⫺30‰ (Vogel, 1993). However, abundant grasses and other arid-region photosynthesizers in this area have probably contributed large amounts of CO2 from C4 (⫺8‰ to ⫺18‰) and CAM (⫺13‰ to ⫺28‰) plants. Cerling and Quade (1993) suggested that coupling between variations in ␦13C and ␦18O should occur in samples of pedogenic calcite taken from various geographic regions. In warmer climates, precipitation is more 18O enriched and caliche that forms there should therefore be likewise enriched in 18O relative to cooler climate examples. Warmer climates also have more C4 plants which yield photosynthetic C more enriched in 13 C than cooler climate C3 plants. Subsequent compilations of published data (Cerling and Quade, 1993) confirm that some regions such as the midwestern and western USA do show this general co-variation, but others do not. Schlesinger (1985) suggested that any co-variation may be due to evaporative enrichment of 18O soil water during warmer times when biologic production of low 13C soil CO2 is retarded and the soil atmosphere contains a higher proportion of atmospheric CO2. The Flagstaff samples are all from the same area, so any co-variations in the older, pedogenic examples due to these proposed effects could relate to climate changes in the 1 to 3 Ma time period represented by the pedogenic samples developed on the older flows. Detailed sampling within the top meter of individual soil profiles developed on limestone and schist revealed co-variant decreases in 18O and 13C in 2 of 3 localities in the southwestern USA (Quade et al., 1989). The topmost caliche layer had ␦13C as high as ⫹6‰, and this decreased within 30 cm to uniform values ⱕ⫺6‰ as determined by the relative amounts of C4 and C3 plants in the area. The decrease in ␦18O with depth was explained as due either to evaporative enrichment in the up-
permost layers or preferential infiltration of low 18O winter precipitation. Schlesinger et al. (1998) found a weak (r ⫽ 0.59) correlation between ␦13C and ␦18O for samples at a soil depth of 35 cm in caliche developed on alluvium and argued that the highest ␦13C (⫺2‰) formed when the horizon had the greatest amount of atmospheric CO2 relative to microbially generated soil CO2 and when there had been significant water loss due to evaporation. These studies demonstrate that ␦13C of caliche in soil horizons can change dramatically in less than 40cm of burial as atmospheric CO2 in the pore spaces gets swamped by biologically respired CO2. Inasmuch as all samples from the Tappan, Merriam, and Woodhouse lavas (Fig. 2) are within soil horizons, it is likely that much of the co-variation in ␦18O and ␦13C observed within them likewise represents a transition to soil atmospheres increasingly dominated by microbial CO2 and soil waters less affected by evaporative enrichment in 18O. As a working hypothesis, we suggest that the lower left end of the data array in Figure 2 therefore defines an end-member which is caliche that develops in soil horizons with a “terminal” ␦13C of ⫺6‰ and ␦18O ranging from about ⫹18‰ to ⫹24‰ and is similar to the type of soil carbonate that has been widely studied elsewhere. The subaerial caliche developed on the 900-yr-old flow is isotopically distinct from the pedogenic caliche both here and worldwide. The minimum ␦13C of ⫹4‰ is near the maximum observed elsewhere. The most plausible explanation for the high ␦18O values relative to the soil carbonate is enrichment in 18 O caused by evaporation of thin films and drops of water. The generally sloping data array indicates that there is a concomitant enrichment in 13C associated with evaporation, possibly as much as 7‰ for an 18O enrichment of 4‰. Simultaneous enrichments in 18O and 13C are theoretically possible if calcite precipitates due to rapid loss of CO2 from thin films of H2O (Hendy, 1971), and such enrichments are not unexpected in the case of speleothems where downward percolating ground waters pick up dissolved CO2 from microbial respiration in overlying soils and degas suddenly upon entering the cave atmosphere. However, the vesicular basalt pinnacles holding these samples have no soil and are extremely porous. Calcite precipitation was almost certainly triggered by evaporation after water emerged into the larger cavities and overhangs rather than by sudden release of CO2. Degassing of geothermal CO2 during groundwater discharge has also been suggested as a source of 13C enriched waters (Valero-Garces et al., 1999), but the high ␦18O values of the caliche precludes a geothermal origin. Kinetic isotope effects during rapid degassing are therefore not an easy explanation for the 13C enrichments reported here. Stiller et al. (1985) have reported 13C enrichments of up to 34.9‰ in concentrated brines undergoing evaporation. Although evaporation is probably responsible for calcite precipitation in the subaerial caliche, the large effects reported for brines may be specific to such concentrated solutions and caution must be used in applying this explanation to evaporation of more dilute solutions. In speleothems where such dilute solutions are involved, C isotope equilibrium between dissolved CO2 and atmospheric CO2 is maintained during evaporative precipitation of calcite (Hendy, 1971). The calcite can become strongly enriched in 18O but not in 13C. If the large ␦13C values in caliche are due to evaporation, they must be due
Isotope Geochemistry of Caliche Developed on Basalt
to unknown kinetic effects associated with very rapid evaporation or processes not yet understood or observed elsewhere. Large enrichments in 13C can be produced during acetate fermentation or equilibration with methane, but these mechanisms are highly unlikely in the oxidative environment of the subaerial pinnacles. Rapid freezing of natural bicarbonate ground waters can also produce calcite powders with extreme 13 C and 18O enrichments (Clark and Lauriol, 1992), similar to the enrichments observed here. However, such cryogenic enrichment is an unsatisfactory explanation for these data because it requires that caliche form only during sudden freezing of water emerging from porous basalt after flow through, or over, meters-high pinnacles and mounds in a geographic setting that rarely experiences sub 0°C temperatures. Also, such precipitation produces cryptocrystalline powder within ice, later released through sublimation or melting. The caliche samples are laminated crusts and pendants, not powders or aggregated powders. A simple biologic mechanism could lead to 13C enrichments associated with evapoconcentration of thin water films on basalt surfaces. Lichens and other photosynthesizing communities are widespread at all sample localities. Many are growing on a caliche substrate and preferentially inhabit the relatively enclosed spaces that collect or retain water during and after rainfall. After rains, water films and drops should have HCO3⫺ with ␦13C ⫽ ⫹1 following dissolution of atmospheric CO2. ␦13C of calcite that precipitates initially then has ␦13C ⫽ ⫹2 to ⫹5 (Quade et al., 1989), similar to the lowest values for the Sunset subaerial caliche (Fig. 2). Explosive activity of photosynthesizers is likely during these brief wet periods. C3 photosynthesizers then fix C with ␦13C ⫽ ⫺20‰ (or some similarly light value). This removes 12C from the dissolved C reservoir and ␦13C of the remaining reservoir thereby increases. If, during evaporation, this reservoir does not re-equilibrate with atmospheric CO2 as fast as 12C is biologically removed, then ␦13C of the dissolved C reservoir continually increases. Calcite that precipitates from this reservoir becomes concomitantly enriched in 13C as evaporation proceeds. This process would produce caliche progressively more enriched in both 13C and 18 O as evaporation progresses. Of the mechanisms above, we consider removal of 12C by photosynthesizing communities during evaporation to be the most plausible, although we cannot rule out some kind of kinetic enrichment related to CO2 degassing during evaporation. Whatever the cause, the important result is that caliche that develops on surfaces and within cracks, vesicles, and other air-filled openings on basalt uncovered by soil is enormously enriched in 18O and 13C relative to that which develops in the soil atmosphere where evaporation is retarded and microbial respiration is rampant. As a working hypothesis, we suggest that the broad, sloping array in Figure 2 is therefore a mixing array with a “subaerial” or “atmospheric” end member and a soil or “pedogenic” end-member. Initial caliche deposition within vesicles and other open spaces may thus lie at the upper right end of the array and get overlain by much thicker caliche layers that are deposited after a soil layer develops. This is similar to the effect described by Quade et al. (1989) in which pedogenic caliche itself develops layers with more of an “atmospheric” signature in the top 30cm. The pedogenic carbonates may, themselves, display the type of co-variation described
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by Quade et al. (1989) within 30 cm profiles of well-developed soil. However, in these examples of caliche on basalt, the isotopic variation from sample to sample is very large and more likely reflects the proportion of initially developed subaerial carbonate to that developed subsequently in a soil atmosphere. The issue of how much calcium in caliche is derived from the parent rock versus the amount contributed from windblown carbonate has been much studied because it is crucial to understanding sources and sinks of Ca in the global weathering budget. Sr isotopes have confirmed that dust is an important, if not dominant, source of Ca in pedogenic carbonate developed on parent rocks of all types in the southwestern USA (e.g., Naiman et al., 2000). Studies of caliche developed on young basalts are extremely limited. Data from Naiman et al. (2000) for 3 samples of Stage II soil carbonate pedogenic caliche developed on one of the older flows in the Flagstaff area (south of the area sampled in this study) show that 55 to 67% of the Sr in the caliche was derived in situ from the parent basalt (assuming that 87Sr/86 for wind-blown dust ⫽ 0.711, the value measured for southern Arizona). Thus, there is apparently a significant amount of Ca that is not of wind-blown origin in these young basalts. Sr isotope work has not yet been done for the 13C,18O enriched vug-fills found in the 900 yr-old subaerial, soil-free flow. The issue of whether fresh basalt can rapidly weather in the Flagstaff area and supply Ca for in situ caliche (as observed in the Hawaiian and Australian examples mentioned above) or whether ALL such caliche is dissolved and reprecipitated wind-blown carbonate dust has not been examined. In the presence of a CO2-bearing atmosphere, carbonate is an inevitable weathering product of basalt. Whatever the proportion of in situ versus wind-blown carbonate, caliche develops during weathering of basalt in a way such that the O and C isotopic composition is completely reset into enormous variations that can be used to explore for the presence of past biologic activity.
4. OTHER VOLCANIC FIELDS
To test whether the isotopic variations are likely to be representative of young basaltic volcanic fields in general, samples were obtained from the Uinkaret Plateau/Vulcan’s Throne volcanic area in northwestern Arizona (180 km northwest of Flagstaff, Arizona) and the Sentinel Volcanic Field in southwestern Arizona (130 km southwest of Phoenix, Arizona). Basaltic cones and lavas in both fields are generally ⬃1 to 3 Ma (Reynolds et al., 1986; Fenton et al., 2001) and have been weathered to various degrees. As in the case of the San Francisco Volcanic Field, there are cinder cones and subaerial pinnacles, knobs, and flow lobes as well as flows with extensively developed soils and pedogenic caliche. The elevation and climate for the NW Arizona fields are essentially indistinguishable from that of the San Francisco Field. The Sentinel Field is at a much lower elevation and lies in the heart of the extremely arid Sonoran Desert. Isotopic data for caliche from the NW Arizona Fields, including the slope on a ␦13C-␦18O diagram, are extremely similar to those for the San Francisco Field (Fig. 3). The range of ␦13C (⫺9‰ to ⫹14‰) is somewhat larger and the range of ␦18O is somewhat smaller (⫹19‰ to ⫹29‰). Data for the
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Fig. 3. Isotopic data for caliche on lavas on the Uinkaret Plateau and Vulcan’s Throne, Northwestern Arizona. The data are essentially indistinguishable from those for samples from the San Francisco Volcanic Field.
Sentinel field have a similar range in ␦13C to both these fields, but ␦18O is shifted ⬃5‰ to higher values (Fig. 4). ␦18O of precipitation in the Sentinel area has not been monitored, but the area is ⬃1650 m lower in elevation relative to the other 2 volcanic fields. ␦18O of precipitation displays a so-called “altitude effect” in which it decreases by ⬃0.3‰/100m increase in elevation (from compilation in Clark and Fritz, 1997). Thus, precipitation in the Sentinel Field should, on average, be ⬃5‰ higher than that of the other 2 fields. The altitude effect thus satisfactorily explains the approximately 5‰ enrichment of ␦18O of the Sentinel caliche relative to that of the other 2 fields. ␦13C of the pedogenic end-member of the Sentinel Field is about ⫺4‰ to ⫺5 ‰, slightly higher than that of the other 2 fields. This may represent slightly heavier C introduced by desert CAM vegetation or may be due to less microbial respiration of very light C. In any case, the extreme 13C enrichments for both the NW Arizona and Sentinel Fields occur only in caliche developed on soil-free hand samples from subaerial cinder cones and pinnacles (Figs. 3 and 4). Samples from thick caliche plates and nodules in the soils yield the lowest ␦13C values. The repetition of this pattern for 3 separate volcanic fields suggests that this is likely to be a universal phenomenon for caliche developed on basalt. The isotope variations are
much greater than those observed in strictly pedogenic caliche developed elsewhere. 5. APPLICATION TO SEARCH FOR LIFE ON LAND IN THE PRECAMBRIAN
On Earth, it appears that ␦13C of bicarbonate in the oceans has generally been within ⬃4 ‰ of its present value over time (Schidlowski, 2001). Atmospheric CO2 in C-isotope equilibrium with the oceanic HCO⫺ 3 reservoir should therefore also have remained within ⬃4‰ of its present value. Because of this, caliche that formed on ancient basalts should display isotope systematics similar to those observed here as long as land surfaces were occupied by photosynthesizing communities to imprint the low ␦13C pedogenic values. The results presented here therefore provide another possible approach for evaluating when such photosynthesizers first occupied the land surfaces. Isotopic and paleontologic investigations of Precambrian paleokarsts suggest that humid landscapes were extensively occupied by photosynthesizing communities at least as far back as 1.2 Ga (Beeunas and Knauth, 1985; Horodyski and Knauth, 1994, Kenny and Knauth, 2001). Geochemical evidence hints at such life as far back as 2.6 Ga (Watanabe et al., 2000). Any Precambrian caliche on basalt is likely to have recrystallized in
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Fig. 4. Isotopic data for caliche on basalts of the Sentinel Volcanic Field, Southwestern Arizona. These data are very similar for the other 2 volcanic fields, but are, in general, ⬃5‰ richer in 18O. This enrichment is satisfactorily explained by the approximately 0.3‰/100 m “altitude effect” on ␦18O of precipitation applied to the approximately 1650 m lower elevation of this field relative too that of the other 2 volcanic fields. ␦13C is possibly 2 to 3‰ higher in this desert area and could reflect a greater proportion of CAM vegetation or less production of microbially respired CO2.
response to inevitable heating and water intrusion during its long burial history. Variations in ␦18O are likely to have homogenized or been altered, depending upon the water/rock ratio and temperature of any later alteration. ␦13C variations, however, are likely to have been largely preserved because of the low water/rock ratio for C in later fluids (e.g., Banner and Hanson, 1990). ␦13C variations in paleocaliche on basalt of the magnitude observed in the modern examples (approximately 20‰) would thus be strong evidence of life on land. Opal layers and fibrous quartz are observed in the modern examples, and this can be expected to convert to microquartz with time. Such microquartz is the type of material most favorable for preservation of microfossils, so ancient examples provide a new target for extracting the elusive fossil record and isotopic signature of life on land in the Precambrian, particularly if the caliche groundmass displays large variations in ␦13C. In essence, large 13C variations provide a method for screening samples most suitable for microfossil prospecting. A limitation of this approach is that subaerial basalt flows, like most landsurface features, are subject to erosion and have poor preservation potential in the geologic record. Also, only examples weathered in relatively arid climates are likely to have devel-
oped caliche; more humid weathering tends to remove Ca from the weathering site. Rare occurrences of Precambrian caliche have nevertheless been reported (Kalliokoski, 1986), so a search for examples associated with basalt is warranted. 6. APPLICATION FOR ASTROBIOLOGICAL PROSPECTING ON MARS
The surface of Mars is littered with boulders and flows of probable basaltic composition. One interpretation of geomorphic features on this now frozen planet suggest that Mars had a warm, wet early history with flowing water and even rainfall (Baker, 2001). Ancient crater forms are largely preserved, so rainfall would have to have been rather limited and long periods of humid climate are thus unlikely. However, rainfall may have been sufficient and lasted long enough to initiate caliche formation on the ubiquitous basaltic materials. There is currently considerable interest in searching for past life on Mars that could have developed during this warm, wet interval, and isotopes in caliche provide a possible approach. The martian situation is more complicated because knowledge of the C reservoirs and their history is not known. Strongly negative
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Fig. 5. Schematic representation of possible biosignature in caliche applied to the case of Mars. If caliche found within martian basalt has isotopically zoned layers as shown, the data would suggest initial subaerial caliche development followed by over layering of soil caliche with photosynthesizing microbes present. The absence of life should lead to a horizontal trajectory (no change in ␦13C) if life is absent. The approach is independent of knowledge of ␦13C of the martian atmospheric CO2 reservoir and ␦18O of the water at the time of caliche formation because it depends only on the relative change in ␦13C associated with changes in ␦18O. The ␦–values shown are arbitrary and bizarre to illustrate this point.
␦13C values in caliche are not, in themselves, a biosignature because the atmospheric reservoir could have been strongly negative at the time of weathering. However, any primary layering of the calcite in vugs, vesicles, and fractures is likely preserved locally because Mars has not undergone the long history of deformation and alteration experienced by the tectonically and hydrologically active Earth. ␦13C of preindustrial atmospheric CO2 on Earth was ⫺6.4‰ (Keeling, 1993). ␣ calcite -CO2 over the temperature range 10 °C to 35 °C is 1.0127 to 1.090. Therefore, the first caliche to develop in evaporating water films on subaerial pinnacles of basalt in isotopic equilibrium with atmospheric CO2 should have ␦13C of ⫹2.6‰ to ⫹6.3‰. Based on the data for the 3 volcanic fields, ␦13C can be expected to increase by ⬃8 to 11‰ during subsequent evaporation due either to kinetic effects or photosynthetic removal of 12C from the evaporating water
drops and films. Initial ␦18O of this caliche depends upon temperature and ␦18O of the precipitation and increases as evaporation progresses. This initial caliche is subsequently over-layered by calcite with ␦18O values similar to the initial precipitate on the subaerial basalt, but may have higher or lower values depending upon climatic temperature variations and variations in ␦18O of the precipitation during the long history of soil development. If photosynthesizing organisms are present, ␦13C will take on values much lower than the inorganic value of the initial atmospheric precipitate. Layers deposited over the original atmospheric caliche will thus be strongly depleted in 18O and 13C relative to the initial vug and fracture linings (Fig. 5). It is this spatial layering of lighter calcite over heavier that is caused by biology because a purely evaporative trend would always get heavier. On Mars, therefore, the spatial variation of isotopic variations in any caliche could provide a
Isotope Geochemistry of Caliche Developed on Basalt
biosignature independent of prior knowledge of the C isotope reservoir. It remains possible that ␦13C variations as high as the ⬎20‰ values we have observed can result from purely kinetic isotope effects during evaporation of water films on basalt. Because the effect has been observed in brines (Stiller et al., 1985), huge ␦13C variations alone cannot presently be considered proof of biologic activity. Indeed, the possibility that unknown inorganic kinetic effects can produce isotopic variations is always present in systems of any kind. Also, anaerobic fermentation and other types of past biologic activity on Mars could have produced ␦13C variations that have no terrestrial analog in the oxic environments of terrestrial basalts. For these reasons, it is highly unlikely that there can ever be an unequivocal purely isotopic biosignature of any kind in a limited number of samples from another world. We do note that the absence of C isotope variations in samples displaying a wide variation in ␦18O would be indicative of evaporation in the absence of biologic activity. However, the subsequent over-layering of 13 C and 18O enriched subaerial caliche (whatever the cause of the 13C enrichment) with subsequent, greatly more 13C,18O depleted pedogenic caliche would be strongly suggestive evidence not to be missed that photosynthesizing microbes were present during caliche formation on Mars. Caliche as a target material for astrobiological prospecting on Mars carries the advantage that samples could be widespread. Basaltic materials are apparently everywhere and many are in the form of highly reactive breccia fragments, vesicular clasts, and lava flows. If these materials ever underwent aqueous weathering in the presence of a CO2-rich atmosphere, carbonate will be an inevitable weathering product. Whether physically retained at the site of weathering or transported away and returned by the wind, caliche should develop during weathering. The material is white and should be readily locatable via close-up imaging. It is relatively soft and therefore one of the easiest of rock materials to sample via scraping, drilling, or quarrying. Its relative softness, however, means that it has certainly been removed from exposed surfaces following the sandblasting that occurs during wind-driven dust storms. It should be preserved in sheltered areas such as along fractures, within vesicles, and underneath boulders and clasts. Its complete absence after searching in such places would be suggestive that Mars probably never had a long, warm, wet early period in which early life could have evolved. A search for caliche on Mars seems highly warranted. 7. IMPLICATIONS FOR ALH 84001
Carbonate has already been found within fractures and vugs in martian meteorites of mafic composition. Evidence for microfossils in the carbonate has been advanced for ALH 84001 (McKay et al., 1996). This meteorite has been shock-metamorphosed and the mineralogical and textural nature of the preshocked carbonate is not evident. ␦18O varies from ⫹5.4‰ to ⫹25.3‰ (Valley et al., 1997; Leshlin et al., 1998), a 20‰ range similar to that observed in the terrestrial caliche developed on basalt. Leshlin et al. (1998) argued that the large variation required a high temperature origin, but alternative, low temperature models involving variable 18O enrichments caused by evaporation of playa lakes (McSween and Harvey,
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1998) or flood waters (Warren, 1998) have also been proposed. By analogy with the terrestrial examples, the ␦18O variation is completely compatible with a low-temperature, evaporative origin for normal caliche developed on basic igneous rocks. Caliche that forms in cracks and vugs can contain dissolved carbonate derived via weathering from many meters away and can be transported in by downward percolating waters. It can thus be deposited on fresh, unweathered rocks surfaces, as observed in ALH 84001. Lack of weathering of the wall rock clearly does not preclude a low-temperature, weathering origin for introduction of the carbonate. ␦13C was not determined for the ion microprobe sample spots that yielded the large ␦18O variations (Leshin et al., 1998). Published ␦13C analyses utilizing phosphoric acid extractions range from ⫹16‰ to ⫹55‰ (Romanek et al., 1994; Jull et al., 1995) and do not show a correlation between ␦18O and ␦13C. Jull et al. (1995) note that ␦13C values ⬍ ⫹45‰ may represent contamination of the ⬍1mg-sized samples. At present, there is no indication of a ␦18O-␦13C correlation similar to that observed in the terrestrial caliche. Mineralogically, the shock-metamorphosed martian carbonate is a complex intergrowth of zoned Fe-Ca-Mg phases quite different from the apparently calcite-dominated terrestrial examples analyzed here. However, the detailed mineralogy of caliche developed on terrestrial basalt has never been extensively investigated. Reconnaissance petrographic examination of a limited number of samples analyzed here revealed a complicated array of highly birefringent, intergrown, microcrystalline grains with extremely variable microscopic textures and fabrics. Mg-rich carbonates occur in caliche developed on Hawaiian basalts (Capo et al., 2000; Whipkey et al., 2002). It remains completely possible that chemistries similar to those observed in ALH 84001 could exist in the terrestrial examples. At this point, the main implication of the present results for ALH 84001 is that the ␦18O variation is consistent with a low temperature caliche origin for the carbonate and that high temperature origins or those involving evaporation of playa lakes or flood waters are not required to explain the oxygen isotope data. Acknowledgments—This work was supported by NASA Exobiology grant NAG5-9430 and Astrobiology Grant NCC2-1051. Todd Luther collected and analyzed samples from the volcanic field in NW Arizona. We thank Michelle Moreno for sample preparation and assistance in the lab. Discussion with J. Quade and reviews by T. Cerling, K. Muehlenbachs, and 2 other anonymous reviewers greatly improved the initial manuscript. We salute Bob Clayton on this occasion of his retirement. Paul Knauth is humbly grateful for Bob’s guidance and introduction to the world of stable isotope geochemistry after being hired as an undergraduate hourly worker in the University of Chicago’s stable isotope lab. It was the opportunity of a lifetime which continues to reward in countless ways. Associate editor: L. Walter REFERENCES Baker V. R. (2001) Water and the Martian landscape. Nature 412, 6843., 228 –236. Banner J. L. and Hanson G. N. (1990) Calculation of simultaneous isotopic and trace– element variations during water–rock interaction with applications to carbonate diagenesis. Geochim. Cosmochim. Acta 54, 3123–3137.
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Pergamon
PII S0016-7037(02)01051-7
Isotope Geochemistry of Caliche Developed on Basalt L. PAUL KNAUTH,*,1 MAURO BRILLI,2 and STAN KLONOWSKI1 1 2
Department of Geological Science, Box 85287-1404, Arizona State University, Tempe AZ85287-1404 USA Department of Geological Sciences University of Roma “La Sapienza”P.le A. Moro, 5 - 00185 Roma (Italy) (Received January 30, 2002; accepted in revised form July 15, 2002)
Abstract—Enormous variations in oxygen and carbon isotopes occur in caliche developed on ⬍ 3 Ma basalts in 3 volcanic fields in Arizona, significantly extending the range of ␦18O and ␦13C observed in terrestrial caliche. Within each volcanic field, ␦18O is broadly co-variant with ␦13C and increases as ␦13C increases. The most 18O and 13C enriched samples are for subaerial calcite developed on pinnacles, knobs, and flow lobes that protrude above tephra and soil. The most 18O and 13C depleted samples are for pedogenic carbonate developed in soil atmospheres. The pedogenic caliche has ␦18O fixed by normal precipitation in local meteoric waters at ambient temperatures and has low ␦13C characteristic of microbial soil CO2. Subaerial caliche has formed from 18O-rich evapoconcentrated meteoric waters that dried out on surfaces after local rains. The associated 13C enrichment is due either to removal of 12C by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation. Caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over-layered during burial by calcite carrying the isotopic signature of the soil environment. The large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if Mars had a “warm, wet” early period and photosynthesizing microbes were present in the early soils. The approach can be similarly applied to terrestrial Precambrian paleocaliche in the search for the earliest record of life on land. Large variations reported for ␦18O of carbonate in Martian meteorite ALH84001 do not necessarily require high temperatures, playa lakes, or flood runoff if the carbonate is an example of altered martian caliche. Copyright © 2003 Elsevier Science Ltd Aside from general interest in caliche on basalt as a terrestrial weathering phenomenon, a better understanding of its isotope systematics could provide a relatively simple strategy for assessing whether aqueous weathering ever occurred on Mars. If Mars had a “warm, wet” early period (e.g., Baker, 2001), then carbonate would have been produced during weathering via the Urey reaction. This weathering product could remain present today as caliche on and within some of the ubiquitous basalt boulder fields (whether retained in situ at the site of weathering or formed from the wind-blown weathering products of basalts in more distant areas). Wind abrasion at rates as high as 0.21 mm/yr (Greeley et al., 1982) has likely removed any exposed caliche on surfaces, but this white material may currently exist in protected cracks, internal vesicles, and under rocks where it could be readily recognized and sampled. On Earth, the oxygen isotopes are fixed at the time of caliche precipitation and are governed by the temperature and isotopic composition of the local meteoric waters. The C isotope composition is determined by the ratio of atmospheric CO2 to soil CO2 produced by microbes (Cerling, 1984). Caliche forms on recent basalt-flow surfaces that are populated by an abundance of lichens, plants, mosses, and microorganisms. It visibly encrusts roots and almost certainly entombs pollens and microbes. The possibility that microbial life was present on Mars during the suggested “warm, wet early” period is currently being seriously explored (e.g., Farmer, 1998). Because of its intimate association with microbial life, any caliche on Mars could well have preserved isotopic (if not microfossil) evidence of any such past life. The isotope geochemistry of carbonate developed in soils
1. INTRODUCTION
In arid regions on Earth, masses of microcrystalline carbonate known as “caliche” are found in vesicles, along fractures, and/or coating surfaces in young basalt flows and cinder cones (Fig. 1). It can become thickly and extensively developed throughout the basalt in soils and in the shallow subsurface. Caliche is common in arid-region soils developed on rocks of all types where it apparently originates mostly by dissolution and reprecipitation of wind-blown carbonate dust (Brown, 1956; Pe´we´ et al., 1981; Capo and Chadwick, 1999). Sr isotope studies show caliche found on weathered basalt contains a mixture of Ca derived from the basalt and Ca derived from wind-blown dust (Naiman et al., 2000). The amount derived from the basalt ranges from 100% for examples in Hawaii (Capo et al., 2000) to less than 50% in areas where surrounding limestones, playas, or sea spray contribute large amounts of wind-blown material (Quade et al., 1995; Naiman et al., 2000). All carbonate on Earth is ultimately derived from the weathering of silicate rocks via reactions generalized in terms of the famous “Urey Reaction” (Urey, 1952): CO2 ⫹ CaSiO3 ⫽ CaCO3 ⫹ SiO2 For basalt, CaSiO3 is a proxy for Ca, Mg silicates such as plagioclase and pyroxene. Whether derived in situ or by recycling of wind-blown carbonate, the oxygen and carbon isotopic composition of caliche has repeatedly been shown to be completely reset during weathering to values determined by the local conditions of weathering (e.g., Cerling and Quade, 1993). * Author to whom ([email protected]).
correspondence
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isotope variations in terrestrial caliche and argue that a strong isotopic biosignature is present in caliche that develops on basalt. The results carry implications for astrobiological prospecting on Mars and also apply to the search for life on land in the Precambrian and to interpretations of stable isotope variations in carbonate in the martian meteorite ALH 84001. 2. SAMPLES, PROCEDURES, AND NOTATION
Fig. 1. A. White caliche coating a vug exposed on the side of a pinnacle of basalt on the 900 yr– old Kannanna Basalt Flow, Sunset Crater, Arizona. Botyroidal microcrystalline calcite is most thickly developed along the top of the cavity. Pocketknife is 6 cm. Locality for sample #755. B. White caliche coating fracture surfaces on older (1 to 3Ma) flow in the San Francisco Volcanic Field, Arizona. A large block was removed for the road cut and exposed caliche– coated fracture surfaces. Hammer is 36 cm. Locality for sample #686. C. Road cut exposure of pedogenic caliche (sample #742) developed in heavily weathered basalt. Hammer handle length is 36 cm. Locality for sample #740.
has been extensively studied (see review by Mermut et al., 2000), but caliche developed specifically on young basalt flows has received very little attention. Consequently, we investigated samples from 3 young volcanic fields in Arizona that contain basaltic lavas ranging in age from 900 yr to ⬃3 Ma. We report data that significantly extend the range of C and O
Field examination revealed two basic types of caliche: 1) Micro-layered and botyroidal accumulations in open spaces such as fractures, amygdules, and cavities developed on cinder cones and flow-deformed mounds of solidified lava; and 2) classic pedogenic accumulations in clay-rich soils developed on the older, more weathered flows. The pedogenic caliche includes indurated, layered horizons and cross-cutting veins of calcite and calcite-cemented basalt rubble. For the purposes of this study, the important distinction is between calcite that has formed in a soil atmosphere (“pedogenic”) versus that which has precipitated in open spaces on pinnacles and lobes of relatively unweathered flows and cones which stand above the local tephra and/or soil (“subaerial”). No attempt was made in this initial survey to define soil types and sample systematically within soil horizons. The samples include the complete spectrum of caliche readily observed in 3 separate volcanic fields. Global Positioning System coordinates of all samples are given in (Electronic Supplement, ES3277, www.elsevier.com) Table 1. 10mg to 20 mg samples were quarried, drilled-out, or scraped from hand samples and analyzed with the original off-line green phosphoric acid technique of McCrea (1950). CO2 so liberated was analyzed on a 15 cm Nier isotope ratio mass spectrometer and corrected for instrumental effects (Craig, 1957). XRD analysis of several typical samples from the older flows confirmed that the pedogenic carbonate was exclusively calcite. Mg carbonates and even dolomite have developed by the direct weathering of young basalts in Hawaii (Capo et al., 2000; Whipkey, 2002), so it is possible that some of our “subaerial” carbonates on the younger flows have a more complicated mineralogy. Oxalates and sulfates are also possible, but none were detected in the limited number of XRD analyses. The phosphoric acid correction of Sharma and Clayton (1965) for calcite was used so that the standard ␦18O notation here refers to lattice oxygen. ␦18O and ␦13C were referenced against lab standards calibrated against the original PDB standard. ␦-values obtained in this fashion are within experimental error (⫾0.2‰ for O and ⫾0.1‰ for C) of those referenced to VPDB. ␦18O values were converted to the SMOW scale using 1.0412 for the CO2-H2O fractionation (O’Neil et al., 1975). All data are given in Table 1. 3. SAN FRANCISCO VOLCANIC FIELD, ARIZONA
The 900 yr-old Kananna basaltic lava flow emerges to the northeast from Sunset Crater (22 km northeast of Flagstaff, Arizona) and lies on a stack of successively older flows as old as 3 Ma (Moore and Wolfe, 1987). Subaerial caliche on the very young Kananna flow is sparsely developed and requires considerable searching to locate. However, it locally reaches thicknesses up to 1 cm along fractures and especially on un-
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Fig. 2. Isotopic data for caliche in the San Francisco Volcanic Field near Flagstaff, Arizona. The data– envelopes are for various stratigraphic ages of the host lavas as mapped by Moore and Wolfe (1987). The most 13C and 18O enriched samples are from pinnacles of a 900 yr– old flow that have never been buried and exposed to a soil atmosphere. The most 13C and 18 O depleted samples occur in thickly developed pedogenic caliche.
derlying surfaces in spaces probably created during flow deformation of solidifying lava (Fig. 1A). It is thickly and widely developed within vesicles and along fractures in the older flows (Fig. 1B). Soils are extensively developed on the older flows throughout the volcanic field and these contain abundant beds, plates, and cross cutting veins of pedogenic caliche (Fig. 1C). Sampling focused on localities near the north end of the Kananna flow where lavas of multiple ages are exposed within a strip 2 km X 15 km (Moore and Wolfe, 1987). Other samples of pedogenic caliche on the older flows were obtained from areas several km to the southeast. Isotopic data for the San Francisco Volcanic Field fall into a broad, sloping array on a ␦13C -␦18O plot in which caliche developed on the youngest lavas is strongly enriched in 18O and 13 C relative to that developed on the older lavas (Fig. 2). The isotopic variations of 18‰ in both ␦18O and ␦13C are truly enormous; samples from this small geographic area display isotopic variations that rival or exceed the range of marine and terrestrial limestones and dolostones, all taken together. ␦13C values rise to ⫹10‰, whereas pedogenic carbonate worldwide almost never exceeds ␦13C values of ⫹5‰. ␦18O is as large as ⫹31.6‰, which is also higher than nearly all published values
for pedogenic carbonate in soils developed on parent rocks of all types. In terms of isotopic composition, this represents a new type of 13C and 18O enriched caliche not reported previously. The most 13C and 18O enriched samples are from the sparsely developed caliche developed on subaerial pinnacles and mounds of 900 yr old lava which is completely free of recognizable soil development and has never been buried and exposed to a soil atmosphere (Fig. 1A). The most 13C and 18O depleted samples are pedogenic caliche developed in the soils of the deeply weathered older flows (“Woodhouse” age; Fig. 1B and 1C). Samples from lavas with intermediate ages (Tappan, Merriam) generally have ␦-values intermediate with respect to oldest and youngest lavas. 3.1. Co-variation of ␦18O and ␦13C in Caliche Oxygen and hydrogen isotopic measurements of precipitation over a 13 yr period at Flagstaff, Arizona, within ⬃300m elevation and within 35 km of the sample areas reveal a mean ␦18O of ⫺8.1‰ (IAEA, 1981). Yearly means vary by over 15‰. The mean temperature during this interval was 7.5°C
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(IAEA, 1981). Calcite in isotopic equilibrium with water with ␦18O ⫽ ⫺8.1‰ at 7.5°C is ⫹24.6‰ (using the CaCO3-H2O fractionation of O’Neil et al., 1969). However, this area is covered with snow for long periods during the winter and it is likely that caliche formation proceeds preferentially during warmer times of the year. At 22°C, ␦18O of the caliche would be ⫹21‰, a value characteristic of the lower left end of the broad sloping array in Figure 2. Soil water is likely to have an oxygen isotopic composition that approximates the long term mean value for the local precipitation, so we interpret the lower left end of the array to represent caliche that formed in soil waters during the non-winter months at temperatures of ⬃7 to 22°C. Pedogenic samples with higher and lower ␦18O values have formed at higher or lower temperatures, respectively, and/or during periods when precipitation had higher or lower ␦18O values than average. Caliche developed within soils of the older flows has negative ␦13C-values ranging from ⬃0‰ to ⫺6‰. These are similar to other soil carbonates worldwide that have precipitated in the presence of respired CO2 modified by diffusive fractionation and mixing with atmospheric CO2 (Salomons et al., 1978; Cerling et al., 1991). At temperatures of 7 to 22°C, ␦13C of soil CO2 in isotopic equilibrium with such calcite of 0‰ to ⫺6‰ is approximately ⫺19‰ to ⫺11‰ using Bottinga (1968) and ⫺17‰ to ⫺9‰ using Romanik et al. (1992). The most negative calcite ␦13C values likely represent soil CO2 with the highest component of respired CO2. The value of ⫺19‰ is higher than expected for C3 plants which normally range from about ⫺20‰ to ⫺30‰ (Vogel, 1993). However, abundant grasses and other arid-region photosynthesizers in this area have probably contributed large amounts of CO2 from C4 (⫺8‰ to ⫺18‰) and CAM (⫺13‰ to ⫺28‰) plants. Cerling and Quade (1993) suggested that coupling between variations in ␦13C and ␦18O should occur in samples of pedogenic calcite taken from various geographic regions. In warmer climates, precipitation is more 18O enriched and caliche that forms there should therefore be likewise enriched in 18O relative to cooler climate examples. Warmer climates also have more C4 plants which yield photosynthetic C more enriched in 13 C than cooler climate C3 plants. Subsequent compilations of published data (Cerling and Quade, 1993) confirm that some regions such as the midwestern and western USA do show this general co-variation, but others do not. Schlesinger (1985) suggested that any co-variation may be due to evaporative enrichment of 18O soil water during warmer times when biologic production of low 13C soil CO2 is retarded and the soil atmosphere contains a higher proportion of atmospheric CO2. The Flagstaff samples are all from the same area, so any co-variations in the older, pedogenic examples due to these proposed effects could relate to climate changes in the 1 to 3 Ma time period represented by the pedogenic samples developed on the older flows. Detailed sampling within the top meter of individual soil profiles developed on limestone and schist revealed co-variant decreases in 18O and 13C in 2 of 3 localities in the southwestern USA (Quade et al., 1989). The topmost caliche layer had ␦13C as high as ⫹6‰, and this decreased within 30 cm to uniform values ⱕ⫺6‰ as determined by the relative amounts of C4 and C3 plants in the area. The decrease in ␦18O with depth was explained as due either to evaporative enrichment in the up-
permost layers or preferential infiltration of low 18O winter precipitation. Schlesinger et al. (1998) found a weak (r ⫽ 0.59) correlation between ␦13C and ␦18O for samples at a soil depth of 35 cm in caliche developed on alluvium and argued that the highest ␦13C (⫺2‰) formed when the horizon had the greatest amount of atmospheric CO2 relative to microbially generated soil CO2 and when there had been significant water loss due to evaporation. These studies demonstrate that ␦13C of caliche in soil horizons can change dramatically in less than 40cm of burial as atmospheric CO2 in the pore spaces gets swamped by biologically respired CO2. Inasmuch as all samples from the Tappan, Merriam, and Woodhouse lavas (Fig. 2) are within soil horizons, it is likely that much of the co-variation in ␦18O and ␦13C observed within them likewise represents a transition to soil atmospheres increasingly dominated by microbial CO2 and soil waters less affected by evaporative enrichment in 18O. As a working hypothesis, we suggest that the lower left end of the data array in Figure 2 therefore defines an end-member which is caliche that develops in soil horizons with a “terminal” ␦13C of ⫺6‰ and ␦18O ranging from about ⫹18‰ to ⫹24‰ and is similar to the type of soil carbonate that has been widely studied elsewhere. The subaerial caliche developed on the 900-yr-old flow is isotopically distinct from the pedogenic caliche both here and worldwide. The minimum ␦13C of ⫹4‰ is near the maximum observed elsewhere. The most plausible explanation for the high ␦18O values relative to the soil carbonate is enrichment in 18 O caused by evaporation of thin films and drops of water. The generally sloping data array indicates that there is a concomitant enrichment in 13C associated with evaporation, possibly as much as 7‰ for an 18O enrichment of 4‰. Simultaneous enrichments in 18O and 13C are theoretically possible if calcite precipitates due to rapid loss of CO2 from thin films of H2O (Hendy, 1971), and such enrichments are not unexpected in the case of speleothems where downward percolating ground waters pick up dissolved CO2 from microbial respiration in overlying soils and degas suddenly upon entering the cave atmosphere. However, the vesicular basalt pinnacles holding these samples have no soil and are extremely porous. Calcite precipitation was almost certainly triggered by evaporation after water emerged into the larger cavities and overhangs rather than by sudden release of CO2. Degassing of geothermal CO2 during groundwater discharge has also been suggested as a source of 13C enriched waters (Valero-Garces et al., 1999), but the high ␦18O values of the caliche precludes a geothermal origin. Kinetic isotope effects during rapid degassing are therefore not an easy explanation for the 13C enrichments reported here. Stiller et al. (1985) have reported 13C enrichments of up to 34.9‰ in concentrated brines undergoing evaporation. Although evaporation is probably responsible for calcite precipitation in the subaerial caliche, the large effects reported for brines may be specific to such concentrated solutions and caution must be used in applying this explanation to evaporation of more dilute solutions. In speleothems where such dilute solutions are involved, C isotope equilibrium between dissolved CO2 and atmospheric CO2 is maintained during evaporative precipitation of calcite (Hendy, 1971). The calcite can become strongly enriched in 18O but not in 13C. If the large ␦13C values in caliche are due to evaporation, they must be due
Isotope Geochemistry of Caliche Developed on Basalt
to unknown kinetic effects associated with very rapid evaporation or processes not yet understood or observed elsewhere. Large enrichments in 13C can be produced during acetate fermentation or equilibration with methane, but these mechanisms are highly unlikely in the oxidative environment of the subaerial pinnacles. Rapid freezing of natural bicarbonate ground waters can also produce calcite powders with extreme 13 C and 18O enrichments (Clark and Lauriol, 1992), similar to the enrichments observed here. However, such cryogenic enrichment is an unsatisfactory explanation for these data because it requires that caliche form only during sudden freezing of water emerging from porous basalt after flow through, or over, meters-high pinnacles and mounds in a geographic setting that rarely experiences sub 0°C temperatures. Also, such precipitation produces cryptocrystalline powder within ice, later released through sublimation or melting. The caliche samples are laminated crusts and pendants, not powders or aggregated powders. A simple biologic mechanism could lead to 13C enrichments associated with evapoconcentration of thin water films on basalt surfaces. Lichens and other photosynthesizing communities are widespread at all sample localities. Many are growing on a caliche substrate and preferentially inhabit the relatively enclosed spaces that collect or retain water during and after rainfall. After rains, water films and drops should have HCO3⫺ with ␦13C ⫽ ⫹1 following dissolution of atmospheric CO2. ␦13C of calcite that precipitates initially then has ␦13C ⫽ ⫹2 to ⫹5 (Quade et al., 1989), similar to the lowest values for the Sunset subaerial caliche (Fig. 2). Explosive activity of photosynthesizers is likely during these brief wet periods. C3 photosynthesizers then fix C with ␦13C ⫽ ⫺20‰ (or some similarly light value). This removes 12C from the dissolved C reservoir and ␦13C of the remaining reservoir thereby increases. If, during evaporation, this reservoir does not re-equilibrate with atmospheric CO2 as fast as 12C is biologically removed, then ␦13C of the dissolved C reservoir continually increases. Calcite that precipitates from this reservoir becomes concomitantly enriched in 13C as evaporation proceeds. This process would produce caliche progressively more enriched in both 13C and 18 O as evaporation progresses. Of the mechanisms above, we consider removal of 12C by photosynthesizing communities during evaporation to be the most plausible, although we cannot rule out some kind of kinetic enrichment related to CO2 degassing during evaporation. Whatever the cause, the important result is that caliche that develops on surfaces and within cracks, vesicles, and other air-filled openings on basalt uncovered by soil is enormously enriched in 18O and 13C relative to that which develops in the soil atmosphere where evaporation is retarded and microbial respiration is rampant. As a working hypothesis, we suggest that the broad, sloping array in Figure 2 is therefore a mixing array with a “subaerial” or “atmospheric” end member and a soil or “pedogenic” end-member. Initial caliche deposition within vesicles and other open spaces may thus lie at the upper right end of the array and get overlain by much thicker caliche layers that are deposited after a soil layer develops. This is similar to the effect described by Quade et al. (1989) in which pedogenic caliche itself develops layers with more of an “atmospheric” signature in the top 30cm. The pedogenic carbonates may, themselves, display the type of co-variation described
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by Quade et al. (1989) within 30 cm profiles of well-developed soil. However, in these examples of caliche on basalt, the isotopic variation from sample to sample is very large and more likely reflects the proportion of initially developed subaerial carbonate to that developed subsequently in a soil atmosphere. The issue of how much calcium in caliche is derived from the parent rock versus the amount contributed from windblown carbonate has been much studied because it is crucial to understanding sources and sinks of Ca in the global weathering budget. Sr isotopes have confirmed that dust is an important, if not dominant, source of Ca in pedogenic carbonate developed on parent rocks of all types in the southwestern USA (e.g., Naiman et al., 2000). Studies of caliche developed on young basalts are extremely limited. Data from Naiman et al. (2000) for 3 samples of Stage II soil carbonate pedogenic caliche developed on one of the older flows in the Flagstaff area (south of the area sampled in this study) show that 55 to 67% of the Sr in the caliche was derived in situ from the parent basalt (assuming that 87Sr/86 for wind-blown dust ⫽ 0.711, the value measured for southern Arizona). Thus, there is apparently a significant amount of Ca that is not of wind-blown origin in these young basalts. Sr isotope work has not yet been done for the 13C,18O enriched vug-fills found in the 900 yr-old subaerial, soil-free flow. The issue of whether fresh basalt can rapidly weather in the Flagstaff area and supply Ca for in situ caliche (as observed in the Hawaiian and Australian examples mentioned above) or whether ALL such caliche is dissolved and reprecipitated wind-blown carbonate dust has not been examined. In the presence of a CO2-bearing atmosphere, carbonate is an inevitable weathering product of basalt. Whatever the proportion of in situ versus wind-blown carbonate, caliche develops during weathering of basalt in a way such that the O and C isotopic composition is completely reset into enormous variations that can be used to explore for the presence of past biologic activity.
4. OTHER VOLCANIC FIELDS
To test whether the isotopic variations are likely to be representative of young basaltic volcanic fields in general, samples were obtained from the Uinkaret Plateau/Vulcan’s Throne volcanic area in northwestern Arizona (180 km northwest of Flagstaff, Arizona) and the Sentinel Volcanic Field in southwestern Arizona (130 km southwest of Phoenix, Arizona). Basaltic cones and lavas in both fields are generally ⬃1 to 3 Ma (Reynolds et al., 1986; Fenton et al., 2001) and have been weathered to various degrees. As in the case of the San Francisco Volcanic Field, there are cinder cones and subaerial pinnacles, knobs, and flow lobes as well as flows with extensively developed soils and pedogenic caliche. The elevation and climate for the NW Arizona fields are essentially indistinguishable from that of the San Francisco Field. The Sentinel Field is at a much lower elevation and lies in the heart of the extremely arid Sonoran Desert. Isotopic data for caliche from the NW Arizona Fields, including the slope on a ␦13C-␦18O diagram, are extremely similar to those for the San Francisco Field (Fig. 3). The range of ␦13C (⫺9‰ to ⫹14‰) is somewhat larger and the range of ␦18O is somewhat smaller (⫹19‰ to ⫹29‰). Data for the
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Fig. 3. Isotopic data for caliche on lavas on the Uinkaret Plateau and Vulcan’s Throne, Northwestern Arizona. The data are essentially indistinguishable from those for samples from the San Francisco Volcanic Field.
Sentinel field have a similar range in ␦13C to both these fields, but ␦18O is shifted ⬃5‰ to higher values (Fig. 4). ␦18O of precipitation in the Sentinel area has not been monitored, but the area is ⬃1650 m lower in elevation relative to the other 2 volcanic fields. ␦18O of precipitation displays a so-called “altitude effect” in which it decreases by ⬃0.3‰/100m increase in elevation (from compilation in Clark and Fritz, 1997). Thus, precipitation in the Sentinel Field should, on average, be ⬃5‰ higher than that of the other 2 fields. The altitude effect thus satisfactorily explains the approximately 5‰ enrichment of ␦18O of the Sentinel caliche relative to that of the other 2 fields. ␦13C of the pedogenic end-member of the Sentinel Field is about ⫺4‰ to ⫺5 ‰, slightly higher than that of the other 2 fields. This may represent slightly heavier C introduced by desert CAM vegetation or may be due to less microbial respiration of very light C. In any case, the extreme 13C enrichments for both the NW Arizona and Sentinel Fields occur only in caliche developed on soil-free hand samples from subaerial cinder cones and pinnacles (Figs. 3 and 4). Samples from thick caliche plates and nodules in the soils yield the lowest ␦13C values. The repetition of this pattern for 3 separate volcanic fields suggests that this is likely to be a universal phenomenon for caliche developed on basalt. The isotope variations are
much greater than those observed in strictly pedogenic caliche developed elsewhere. 5. APPLICATION TO SEARCH FOR LIFE ON LAND IN THE PRECAMBRIAN
On Earth, it appears that ␦13C of bicarbonate in the oceans has generally been within ⬃4 ‰ of its present value over time (Schidlowski, 2001). Atmospheric CO2 in C-isotope equilibrium with the oceanic HCO⫺ 3 reservoir should therefore also have remained within ⬃4‰ of its present value. Because of this, caliche that formed on ancient basalts should display isotope systematics similar to those observed here as long as land surfaces were occupied by photosynthesizing communities to imprint the low ␦13C pedogenic values. The results presented here therefore provide another possible approach for evaluating when such photosynthesizers first occupied the land surfaces. Isotopic and paleontologic investigations of Precambrian paleokarsts suggest that humid landscapes were extensively occupied by photosynthesizing communities at least as far back as 1.2 Ga (Beeunas and Knauth, 1985; Horodyski and Knauth, 1994, Kenny and Knauth, 2001). Geochemical evidence hints at such life as far back as 2.6 Ga (Watanabe et al., 2000). Any Precambrian caliche on basalt is likely to have recrystallized in
Isotope Geochemistry of Caliche Developed on Basalt
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Fig. 4. Isotopic data for caliche on basalts of the Sentinel Volcanic Field, Southwestern Arizona. These data are very similar for the other 2 volcanic fields, but are, in general, ⬃5‰ richer in 18O. This enrichment is satisfactorily explained by the approximately 0.3‰/100 m “altitude effect” on ␦18O of precipitation applied to the approximately 1650 m lower elevation of this field relative too that of the other 2 volcanic fields. ␦13C is possibly 2 to 3‰ higher in this desert area and could reflect a greater proportion of CAM vegetation or less production of microbially respired CO2.
response to inevitable heating and water intrusion during its long burial history. Variations in ␦18O are likely to have homogenized or been altered, depending upon the water/rock ratio and temperature of any later alteration. ␦13C variations, however, are likely to have been largely preserved because of the low water/rock ratio for C in later fluids (e.g., Banner and Hanson, 1990). ␦13C variations in paleocaliche on basalt of the magnitude observed in the modern examples (approximately 20‰) would thus be strong evidence of life on land. Opal layers and fibrous quartz are observed in the modern examples, and this can be expected to convert to microquartz with time. Such microquartz is the type of material most favorable for preservation of microfossils, so ancient examples provide a new target for extracting the elusive fossil record and isotopic signature of life on land in the Precambrian, particularly if the caliche groundmass displays large variations in ␦13C. In essence, large 13C variations provide a method for screening samples most suitable for microfossil prospecting. A limitation of this approach is that subaerial basalt flows, like most landsurface features, are subject to erosion and have poor preservation potential in the geologic record. Also, only examples weathered in relatively arid climates are likely to have devel-
oped caliche; more humid weathering tends to remove Ca from the weathering site. Rare occurrences of Precambrian caliche have nevertheless been reported (Kalliokoski, 1986), so a search for examples associated with basalt is warranted. 6. APPLICATION FOR ASTROBIOLOGICAL PROSPECTING ON MARS
The surface of Mars is littered with boulders and flows of probable basaltic composition. One interpretation of geomorphic features on this now frozen planet suggest that Mars had a warm, wet early history with flowing water and even rainfall (Baker, 2001). Ancient crater forms are largely preserved, so rainfall would have to have been rather limited and long periods of humid climate are thus unlikely. However, rainfall may have been sufficient and lasted long enough to initiate caliche formation on the ubiquitous basaltic materials. There is currently considerable interest in searching for past life on Mars that could have developed during this warm, wet interval, and isotopes in caliche provide a possible approach. The martian situation is more complicated because knowledge of the C reservoirs and their history is not known. Strongly negative
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Fig. 5. Schematic representation of possible biosignature in caliche applied to the case of Mars. If caliche found within martian basalt has isotopically zoned layers as shown, the data would suggest initial subaerial caliche development followed by over layering of soil caliche with photosynthesizing microbes present. The absence of life should lead to a horizontal trajectory (no change in ␦13C) if life is absent. The approach is independent of knowledge of ␦13C of the martian atmospheric CO2 reservoir and ␦18O of the water at the time of caliche formation because it depends only on the relative change in ␦13C associated with changes in ␦18O. The ␦–values shown are arbitrary and bizarre to illustrate this point.
␦13C values in caliche are not, in themselves, a biosignature because the atmospheric reservoir could have been strongly negative at the time of weathering. However, any primary layering of the calcite in vugs, vesicles, and fractures is likely preserved locally because Mars has not undergone the long history of deformation and alteration experienced by the tectonically and hydrologically active Earth. ␦13C of preindustrial atmospheric CO2 on Earth was ⫺6.4‰ (Keeling, 1993). ␣ calcite -CO2 over the temperature range 10 °C to 35 °C is 1.0127 to 1.090. Therefore, the first caliche to develop in evaporating water films on subaerial pinnacles of basalt in isotopic equilibrium with atmospheric CO2 should have ␦13C of ⫹2.6‰ to ⫹6.3‰. Based on the data for the 3 volcanic fields, ␦13C can be expected to increase by ⬃8 to 11‰ during subsequent evaporation due either to kinetic effects or photosynthetic removal of 12C from the evaporating water
drops and films. Initial ␦18O of this caliche depends upon temperature and ␦18O of the precipitation and increases as evaporation progresses. This initial caliche is subsequently over-layered by calcite with ␦18O values similar to the initial precipitate on the subaerial basalt, but may have higher or lower values depending upon climatic temperature variations and variations in ␦18O of the precipitation during the long history of soil development. If photosynthesizing organisms are present, ␦13C will take on values much lower than the inorganic value of the initial atmospheric precipitate. Layers deposited over the original atmospheric caliche will thus be strongly depleted in 18O and 13C relative to the initial vug and fracture linings (Fig. 5). It is this spatial layering of lighter calcite over heavier that is caused by biology because a purely evaporative trend would always get heavier. On Mars, therefore, the spatial variation of isotopic variations in any caliche could provide a
Isotope Geochemistry of Caliche Developed on Basalt
biosignature independent of prior knowledge of the C isotope reservoir. It remains possible that ␦13C variations as high as the ⬎20‰ values we have observed can result from purely kinetic isotope effects during evaporation of water films on basalt. Because the effect has been observed in brines (Stiller et al., 1985), huge ␦13C variations alone cannot presently be considered proof of biologic activity. Indeed, the possibility that unknown inorganic kinetic effects can produce isotopic variations is always present in systems of any kind. Also, anaerobic fermentation and other types of past biologic activity on Mars could have produced ␦13C variations that have no terrestrial analog in the oxic environments of terrestrial basalts. For these reasons, it is highly unlikely that there can ever be an unequivocal purely isotopic biosignature of any kind in a limited number of samples from another world. We do note that the absence of C isotope variations in samples displaying a wide variation in ␦18O would be indicative of evaporation in the absence of biologic activity. However, the subsequent over-layering of 13 C and 18O enriched subaerial caliche (whatever the cause of the 13C enrichment) with subsequent, greatly more 13C,18O depleted pedogenic caliche would be strongly suggestive evidence not to be missed that photosynthesizing microbes were present during caliche formation on Mars. Caliche as a target material for astrobiological prospecting on Mars carries the advantage that samples could be widespread. Basaltic materials are apparently everywhere and many are in the form of highly reactive breccia fragments, vesicular clasts, and lava flows. If these materials ever underwent aqueous weathering in the presence of a CO2-rich atmosphere, carbonate will be an inevitable weathering product. Whether physically retained at the site of weathering or transported away and returned by the wind, caliche should develop during weathering. The material is white and should be readily locatable via close-up imaging. It is relatively soft and therefore one of the easiest of rock materials to sample via scraping, drilling, or quarrying. Its relative softness, however, means that it has certainly been removed from exposed surfaces following the sandblasting that occurs during wind-driven dust storms. It should be preserved in sheltered areas such as along fractures, within vesicles, and underneath boulders and clasts. Its complete absence after searching in such places would be suggestive that Mars probably never had a long, warm, wet early period in which early life could have evolved. A search for caliche on Mars seems highly warranted. 7. IMPLICATIONS FOR ALH 84001
Carbonate has already been found within fractures and vugs in martian meteorites of mafic composition. Evidence for microfossils in the carbonate has been advanced for ALH 84001 (McKay et al., 1996). This meteorite has been shock-metamorphosed and the mineralogical and textural nature of the preshocked carbonate is not evident. ␦18O varies from ⫹5.4‰ to ⫹25.3‰ (Valley et al., 1997; Leshlin et al., 1998), a 20‰ range similar to that observed in the terrestrial caliche developed on basalt. Leshlin et al. (1998) argued that the large variation required a high temperature origin, but alternative, low temperature models involving variable 18O enrichments caused by evaporation of playa lakes (McSween and Harvey,
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1998) or flood waters (Warren, 1998) have also been proposed. By analogy with the terrestrial examples, the ␦18O variation is completely compatible with a low-temperature, evaporative origin for normal caliche developed on basic igneous rocks. Caliche that forms in cracks and vugs can contain dissolved carbonate derived via weathering from many meters away and can be transported in by downward percolating waters. It can thus be deposited on fresh, unweathered rocks surfaces, as observed in ALH 84001. Lack of weathering of the wall rock clearly does not preclude a low-temperature, weathering origin for introduction of the carbonate. ␦13C was not determined for the ion microprobe sample spots that yielded the large ␦18O variations (Leshin et al., 1998). Published ␦13C analyses utilizing phosphoric acid extractions range from ⫹16‰ to ⫹55‰ (Romanek et al., 1994; Jull et al., 1995) and do not show a correlation between ␦18O and ␦13C. Jull et al. (1995) note that ␦13C values ⬍ ⫹45‰ may represent contamination of the ⬍1mg-sized samples. At present, there is no indication of a ␦18O-␦13C correlation similar to that observed in the terrestrial caliche. Mineralogically, the shock-metamorphosed martian carbonate is a complex intergrowth of zoned Fe-Ca-Mg phases quite different from the apparently calcite-dominated terrestrial examples analyzed here. However, the detailed mineralogy of caliche developed on terrestrial basalt has never been extensively investigated. Reconnaissance petrographic examination of a limited number of samples analyzed here revealed a complicated array of highly birefringent, intergrown, microcrystalline grains with extremely variable microscopic textures and fabrics. Mg-rich carbonates occur in caliche developed on Hawaiian basalts (Capo et al., 2000; Whipkey et al., 2002). It remains completely possible that chemistries similar to those observed in ALH 84001 could exist in the terrestrial examples. At this point, the main implication of the present results for ALH 84001 is that the ␦18O variation is consistent with a low temperature caliche origin for the carbonate and that high temperature origins or those involving evaporation of playa lakes or flood waters are not required to explain the oxygen isotope data. Acknowledgments—This work was supported by NASA Exobiology grant NAG5-9430 and Astrobiology Grant NCC2-1051. Todd Luther collected and analyzed samples from the volcanic field in NW Arizona. We thank Michelle Moreno for sample preparation and assistance in the lab. Discussion with J. Quade and reviews by T. Cerling, K. Muehlenbachs, and 2 other anonymous reviewers greatly improved the initial manuscript. We salute Bob Clayton on this occasion of his retirement. Paul Knauth is humbly grateful for Bob’s guidance and introduction to the world of stable isotope geochemistry after being hired as an undergraduate hourly worker in the University of Chicago’s stable isotope lab. It was the opportunity of a lifetime which continues to reward in countless ways. Associate editor: L. Walter REFERENCES Baker V. R. (2001) Water and the Martian landscape. Nature 412, 6843., 228 –236. Banner J. L. and Hanson G. N. (1990) Calculation of simultaneous isotopic and trace– element variations during water–rock interaction with applications to carbonate diagenesis. Geochim. Cosmochim. Acta 54, 3123–3137.
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