South Africa) Reflect a Stinking Sulfurous Swamp, Not Atmospheric CO2 ⁎ Neil J

Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 370–381 www.elsevier.com/locate/palaeo

δ13C values of carbonate nodules across the Permian–Triassic boundary in the Karoo Supergroup (South Africa) reflect a stinking sulfurous swamp, not atmospheric CO2 ⁎ Neil J. Tabor a, , Isabel P. Montañez b, Maureen B. Steiner c, Dylan Schwindt c

a Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, United States b Department Geosciences, University of California, Davis, CA 95616, United States c Department Geology and Geophysics, University of Wyoming, Laramie, WY 82071, United States

Accepted 30 November 2006

Abstract

Paleosol macromorphology, petrography and stable isotope geochemistry of carbonate nodules from Permo–Triassic (P–T) boundary strata of the central Karoo basin, South Africa are presented in order to evaluate their utility as geochemical archives of Permian–Triassic paleoatmospheric chemistry and their potential to provide a terrestrial carbon isotope chemostratigraphy for correlation with contemporaneous global marine δ13C records. Paleosol morphologies across P–T boundary strata of the central Karoo basin indicate the region was a poorly drained, seasonally to continuously flooded bottomland. Radiaxial calcite, microspar and micrite microtextures are observed in the calcite nodules associated with P–T paleosol profiles. Micritic calcite is typical of soil carbonate that forms in well-drained soils characterized by open-system gaseous diffusion between the soil and global atmosphere, whereas more coarsely crystalline textures and radiaxial calcite are more typical of crystallization in phreatic environments with limited gaseous diffusion between the soil and global atmosphere. In this regard, only those nodules composed of micritic calcite can be considered as a potentially reliable source of proxy information for paleoatmospheric δ13C values across the P–T boundary. The δ13C values of all carbonate nodules from two overlapping measured sections near Carlton Heights, South Africa range from −24.4‰ to −1.8‰, whereas micritic calcite nodule δ13C values range from −7.0‰ to −21.3‰. Comparison of calcite δ13C values with penecontemporaneous organic matter δ13C values indicate that calcite δ13C valuesb−10.6‰ cannot provide a record of atmospheric δ13C values. Rather, these more negative carbonate nodule δ13C values likely reflect calcite crystallization under poorly drained, swampy conditions characterized by a semi-closed chemical system that evolved independently of Earth's troposphere. Therefore, previously reported negative carbon isotope shift(s) measured from carbonate nodules in the Karoo basin P–T strata do not provide a record of atmospheric δ13C, and probably do not provide a viable carbon isotope stratigraphy that can be realistically correlated with global P–T marine carbon isotope stratigraphies. © 2007 Elsevier B.V. All rights reserved.

Keywords: Permian–Triassic boundary; Paleosols; Carbonate nodules; Isotopes

1. Introduction

The end-Permian extinction is characterized by a 13 ⁎ Corresponding author. Tel.: +1 214 768 4175. large and relatively abrupt, global-scale negative δ C E-mail address: [email protected] (N.J. Tabor). excursion recorded in both marine (2 to −4‰; Holser

0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.11.047 N.J. Tabor et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 370–381 371 and Magaritz, 1992; Twitchett et al., 2001; de Wit et al., nodules in the P–T Karoo strata that (1) excludes any 2002; Sephton et al., 2002) and terrestrial (Sephton significant atmospheric contribution of carbon, (2) pre- et al., 2002; Sarkar et al., 2003) strata. This δ13C excludes previous assertions that Karoo paleosol carbonate cursion is interpreted to reflect major perturbation to nodules provide a chemostratigraphic correlation to the carbon cycling, and possible collapse of primary pro- marine P–T boundary. ductivity, through Earth's near-surface carbon reser- voirs. An ∼−10‰ decrease in carbonate nodule δ13C 2. Methods and background values within the P–T boundary interval of the central Karoo basin, South Africa, has been interpreted as a Two partially overlapping stratigraphic sections record of influx of C12 to atmospheric carbon from mass (Sections 1 and 2), ∼600 m apart, were measured at death and oxidation of primary producers leading up to the Carlton Heights locality, along the Graaff–Reinet and approximately coincident with the paleontologically Colesburg highway between Middleburg and Neupoort, defined extinction, suggesting a mechanistic link be- South Africa (31°13.03′ S, 24°56.96′ E; Fig. 1). The tween changing atmospheric chemistry and the end- Carlton Heights section was specifically selected for this Paleozoic life crisis (MacLeod et al., 2000; Ward et al., study because it is relatively free of the prevalent in- 2005). The significance of this interpretation on our trusions of Jurassic dolerites (Steiner et al., 2003), and understanding of the Late Permian Earth system, and its thus probably experienced a milder diagenetic history impact upon our collective perception of mass extinc- than other P–T boundary sections in the region. tion as it applies to connections between continental and Paleosols were logged and described in detail using marine environments, is contingent upon whether these established criteria (e.g., Tabor and Montañez, 2004) carbonate nodule δ13C values provide an accurate and classified according to the paleosol classification record of P–T atmospheric δ13C values and therefore system of Mack et al. (1993). Thin sections (n=47) of present a viable chemostratigraphic link to the global representative carbonate samples were examined by marine δ13C curve. transmitted light and Scanning Electron Microscopy Here we present morphologic, petrographic and (SEM) to determine micromorphology and mineralogy geochemical data from carbonate nodules associated following the approach of Deutz et al. (2001). with fossil soils developed in P–T strata of the Karoo Carbonate samples (20–60 mg) were collected from basin, South Africa. The results provide the basis for an thick-sections using a low-speed diamond-bit drill alternative origin for the δ13C values of carbonate attached to a binocular microscope. Isotopic analyses

Fig. 1. Location map showing the Carlton Heights, Lootsberg Pass, Bethulie, and Brakfontein localities (Karoo Basin, South Africa) of this study. 372 N.J. Tabor et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 370–381 were performed in the stable isotope laboratory in the Table 1 δ13 δ18 Department of Geology, UC Davis, using a Fisons Texture, location, stratigraphic position, C and O values of carbonate nodules from the Carlton Heights section Optima isotope ratio mass spectrometer equipped with δ13 δ18 δ18 an Isocarb automated carbonate system that produces Meter Texture of Section CPDB OVPDB OVSMOW level a microfabric (‰) (‰) (‰) CO2 by dissolution of sample powders in 105% H3PO4 at 95 °C. Measured isotope values are reported in per 4.5 Micrite 1 −5.6 −13.4 17.1 − − mil notation, where 4.5 Micrite 1 7.1 14.1 16.4 50.0 Microspar 1 −12 −15.4 15.0 50.0 Radiaxial 1 −24.4 −19 11.3 R d13Cðor d18OÞ¼ sample 1 1000 6.5 Microspar 1 −4.9 −12.9 17.6 Rstandard 66.0 Radiaxial 1 −8.8 −13.9 16.6 66.0 Microspar 1 −12.8 −14.6 15.9 − − R= 13C/12Cand18O/16O for carbon and oxygen, res- 113.0 Micrite 1 2.2 9.9 20.7 inclusion pectively. The standard used to report isotope values of 114.0 Micrite 1 −1.8 −10.2 20.4 calcite are the Pee Dee Belemnite (PDB; Craig, 1957) inclusion and standard meteoric ocean water (SMOW; Gonfiantini, 112.0 Radiaxial 1 −7.3 −11 19.6 1984). Replicate analyses of the calcite standard NBS-19 28.0 Micrite 1 −5.7 −11.2 19.4 δ13 δ18 inclusion yielded CPDB values of 1.92± 0.14 and OPDB values − − − ‰ 28.0 Radiaxial 1 9.5 13.3 17.2 of 2.07 ±0.15(n=33) over the analysis period. Results 28.0 Micrite 1 −5.2 −11.6 19.0 of isotope analyses from Karoo basin Permian–Triassic 41.0 Microspar 1 −5.1 −10.5 20.1 carbonate nodules are presented in Table 1. The results are 41.0 Radiaxial 1 −15.5 −13 17.5 presented in the stratigraphic and sedimentologic frame- −21.0 Micrite 2 −21.6 −7 23.7 work defined by previous workers (Smith, 1995; Steiner inclusion −18.8 Micrite 2 −20.6 −2.1 28.8 et al., 2003). inclusion −18.8 Micrite 2 −18.3 −18.2 12.2 3. Lithostratigraphy inclusion −18.8 Radiaxial 2 −16.9 −17.9 12.5 − − A detailed section of Carlton Heights' lithostratigra- 14.5 Radiaxial 2 15.8 16.3 14.1 24.6 Micrite 2 −17.8 −17.1 13.3 phy is shown in Fig. 2. Neil Tabor (the 1st author in this 24.6 Micrite 2 −17.2 −16 14.4 work) originally measured the stratigraphic section in 25.5 Micrite 2 −16.8 −16.1 14.3 the month of July 1999. Aspects of the stratigraphy 25.5 Micrite 2 −21.9 −11.8 18.8 were subsequently presented in Steiner et al. (2003).In 25.5 Microspar 2 −21.2 −8.3 22.4 − − − general, the Permian Balfour Formation consists pri- 16.2 Microspar 2 18.7 21.5 8.8 −16.2 Micrite 2 −18.5 −21.3 9.0 marily of green to very dark red overbank mudstone a deposits interbedded with thin ribbons of overbank fine The 0 m level is arbitrarily placed at the bottom of the measured section 1 at Carlton Heights. sandstones and rare channel sandstones with low angle cross-stratification. Both mudstones and sandstones of the Balfour Formation exhibit subhorizontal to subvertical burrows and subspherical to lenticular carbonate The Katberg Formation at Carlton Heights is more nodules ranging from ∼5–20 mm in diameter. The than 270 m thick. It consists of light greenish-gray to fungal “spike” horizon that represents one of the pro- white, multistoried and laterally extensive fine- to posed Permian–Triassic boundary zones in the Karoo medium-grained sandstones with scoured bases, hori- basin occurs in the upper meter of the Balfour Formation zontal and large-scale trough-cross stratification and at Carlton Heights (Steiner et al., 2003). Note that this basal conglomeratic lag deposits consisting of carbonate horizon occurs between 35.6 and 36.6 m in Fig. 2, nodules, mud rip-up clasts and fossil bone material. The whereas the same horizon occurs between 57.6 and thick sandstones are interbedded with thin red mud- 58.6 m in Steiner et al. (2003). This discrepancy reflects stones that typically exhibit desiccation features sim- that the lower 22 m of the Balfour Formation in Steiner ilar to mud cracks. At several levels within the lower et al. (2003) is taken from a partially overlapping section Katberg Formation, sandstones are intensively bur- (measured by N. Tabor) that was measured ∼600 m to rowed and preserve abundant carbonate nodules. Based the south of the section depicted in Fig. 2. Immediately upon our own and other's work at the Carlton Heights above the fungal spike is the first laterally widespread, section (Gastaldo et al., 2005 and person. commun.), multistoried sandstone of the Katberg Formation. carbonate nodules have been observed to cement both N.J. Tabor et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 370–381 373

Fig. 2. Detailed stratigraphy of Carlton Heights Section 1 from the P–T boundary strata of the Karoo basin, South Africa. Proposed P–T boundary intervals occur between ∼5 m and 17 m according to Ward and Smith (2000) and Ward et al. (2005),at∼17 m according to Retallack et al. (2003),or at ∼36 m according to Steiner et al. (2003). the sandstone matrix and fill within these tubular and infilling of burrows. Nevertheless, carbonate burrows, indicating that some of the carbonate nodules nodules in the intra-formational conglomeratic lag formed in the burial environment, after abandonment deposits likely indicate that some population of 374 N.J. Tabor et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 370–381 carbonate nodules in the Balfour and Katberg Forma- Retallack et al., 2003; Steiner et al., 2003; Ward et al., tions formed very early in the burial environment and 2005). Significantly, MacLeod et al. (2000) reported a were subsequently eroded and reworked into stream large, negative δ13C excursion of about 10‰ measured channels (see also Smith, 1995; Pace et al., 2005). from calcareous nodules in pedogenically-altered hor- Although the fungal-rich horizon at Carlton Heights izons within this laminated mudstone. The negative has not been identified in other sections of the Karoo δ13C excursion was attributed to the global-scale neg- basin, the lithostratigraphic patterns at other important ative carbon isotope excursion documented in Permo– sections, such as Brakfontein, Bethulie, and Lootsberg Triassic marine intervals (e.g., references therein). Pass, are remarkably similar (Smith, 1995; Steiner et al., Based on the lithostratigraphic relationships of the two 2003; see also Groenewald, 1989; Ward et al., 2000; sections, the Bethulie section negative δ13C excursion Smith and Ward, 2001; Retallack et al., 2003; Gastaldo (MacLeod et al., 2000)is∼19 m below the fungal spike et al., 2005; Ward et al., 2005). Green mudstones and horizon at Carlton Heights. Subsequently, Ward et al. overbank deposits dominate the Upper Permian Balfour (2005) presented a diagram of “combined data from Formation in these sections. The green mudstones are carbonate nodules obtained from the Carlton Heights successively overlain by thinly bedded to massive very and the Lootsberg regions”, suggesting that isotopically dark red and green-gray mudstones interbedded with a negative δ13C values do not occur as a spike, but as a few thin tabular sands of the upper Balfour Formation, longer-term event (at least stratigraphically, if not followed by multistoried, laterally widespread sand- temporally) that is represented through several different stones of the Katberg Formation. beds. Unfortunately, no stratigraphic scale was provided The discovery of the fungal spike horizon by Steiner for this figure, so the stratigraphic thickness of the et al. (2003) is acknowledged in Retallack et al. (2003), isotope excursion cannot be absolutely known from that but Retallack and others' study asserts that the Permian– contribution, but it is probably limited to the interval Triassic boundary occurs 17–18 m below the fungal between 5 and 17 m in Fig. 2. spike at the contact between an ∼10 cm thick claystone breccia layer and an overlying thin ripple cross lam- 4. Paleosols and carbonate nodules inated sandstone. This contact was proposed as the Permo–Triassic boundary based on regional lithologi- Smith (1990, 1995) provided some of the earliest cal, paleopedological and paleontological changes accounts of paleosol descriptions from the Permian– across this region of Gondwanaland. We did not observe Triassic strata of the Karoo basin. Smith (1995) re- such lithostratigraphic units 17–18 m below the fungal ported that “…the mudrocks host hydromorphic paleo- spike horizon at Carlton Heights (Fig. 2). However, the sols with palustrine carbonate horizons…” below the Permo–Triassic boundary used by Retallack et al. Permian–Triassic boundary and “…strata change into (2003) might correspond to the sharp and wavy contact reddish-brown mudrocks with numerous nodular hor- between a 105 cm thick red mudstone and an overlying izons and pervasive calcareous encrustation…” in the 120 cm thick unit that coarsens upward from mudstone Triassic rocks above the boundary. Smith (1990, 1995) to fine sandstones with heterolithic cross-stratification noted that carbonate horizons appear as lenticular (5 m and lime-pebble conglomeratic lag deposits 18.6 m long and 10 cm thick) accumulations that mantle ho- below the fungal spike layer in Fig. 2. rizons of illuviated clay. In addition, Smith (1995) The Permo–Triassic boundary layer proposed by interpreted these carbonate horizons to be palustrine, or Retallack et al. (2003) lies within the same Permo– shallow pond and groundwater, deposits. Although Triassic boundary zone proposed by Smith and Ward those earlier studies cited specific evidence to indicate (2001) and Ward et al. (2005). Those studies concluded in-situ carbonate accumulation at or below the position that the Permo–Triassic transition is limited to an ∼7m of the paleo-water table (“palustrine carbonates”), those thick layer of purple and gray laminated mudstones in studies also noted that conglomeratic lag deposits of the upper Palingkloof Member of the Balfour Forma- calcitic nodules likely correspond to formation of car- tion. based on biostratigraphic, magnetostratigraphic bonates in shallow sedimentary profiles above the and chemostratigraphic trends in the Lootsberg Pass groundwater table and reworking of those nodules into (eastern Cape Province) and Bethulie (Orange Free basal channel environments prior to burial beneath the State) sections. Based on lithological characteristics and phreatic zone. stratigraphical relationships, this so called “event bed” is Retallack et al. (2003) described and evaluated correlated with red-beds near the base of the Carlton paleosol profiles across the Permian–Triassic boundary Heights section (from ∼5 m to 17 m in Fig. 2; see also strata at several important sites in the Karoo basin N.J. Tabor et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 370–381 375

including Bethulie, Carlton Heights, and Lootsberg Pass localities. Based upon soil morphological and chemical compositions, Retallack et al. (2003) recognized 12 different sorts of paleosol profiles, or pedotypes. Six of the pedotypes occur only in Permian strata, and the other six pedotypes occur only in Triassic strata. These Permian–Triassic pedotypes correspond to Protosols, Calcisols and Gleysols in the paleosol classification of Mack et al. (1993) and “Such assemblages of paleosols are found in alluvial bottomlands with seasonally fluc- tuating, but locally high water tables in arid to semiarid climates” (Retallack et al., 2003; p. 1141). There are three different Permian pedotypes with carbonate nodules: Bada, Hom and Som. Bada paleosols are “blue-gray in color and have unusually high FeO/Fe2O3 ratios, indicating seasonally inundated (gleyed) soils” (Retallack et al., 2003; p. 1141), and that the gray coloration of the paleosol profiles may record the effects of “stagnant groundwater (gleization)” (Retallack et al., 2003; p. 1140). Interpretations of pedogenic environments of “Hom and Som paleosols are comparable” to those of Bada paleosols (Retallack et al., 2003; p. 1141). There are also three Triassic pedotypes with carbonate nodules in the Karoo basin: Sedibo, Karie and Kuta palesols. “Sedibo paleosols are gray and have iron- manganese nodules as evidence of gleization due to seasonal inundation…” (Retallack et al., 2003). Karie and Kuta paleosols contain layers of illuvial clay, and were interpreted to have formed in well-drained environments (Retallack et al., 2003). Therefore, based on previous studies (Smith, 1990, 1995; Retallack et al., 2003), there is persuasive morphological and chemical evidence to indicate poorly-drained and waterlogged conditions in the majority of calcite-nodule-bearing paleosol profiles through the Permian–Triassic boundary strata of the Karoo basin. We do not use the pedotype approach that was implemented by Retallack et al. (2003). Nevertheless, based upon the morphological and lithological attributes of the Carlton Heights strata, we agree that Protosols, Gleysols and Calcisols are the dominant paleosol morphologies in the Karoo P–T strata. All three paleosol morphologies have mudstone-rich and sandstone-rich end members, although mudstone morphologies are generally more abundant in the Balfour Formation.

Fig. 3. Calcite textures at Carlton Heights. a) Radiaxial calcite, possibly preserved as a pseudomorphic replacement after gypsum rosette. Scale=0.2 mm, crossed-nichols. b) SEM image of calcified gypsum rosettes. Scale is 2 μm. c) Microspar calcite with occluded detrital grains (dark and very bright area). Scale=500 μm, crossed- nichols. d) Radiaxial calcite with stylolitized micrite inclusion. Scale bar is 1 mm, crossed-nichols. 376 N.J. Tabor et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 370–381

Protosols preserve features indicative of disruption of calcites. Dissolution structures such as microstylolites primary sedimentary features; sandy profiles exhibit are common in miciritic textures (Fig 3d). light gray to buff, low-chroma matrix colors with single- grain structure, whereas muddy profiles display green- 4.2. Isotopical analyses ish-gray to very dark red soil matrix colors with massive structure. These paleosols preserve subvertical to sub- Carbonate nodule δ13C and δ18O values from Sec- horizontal sand-filled burrows and tubular structures tions 1 and 2 at Carlton Heights are presented in Fig. 4 13 18 resembling root traces. Gleysols are massive to weak and Table 1. δ CPDB and δ OSMOW values range from angular blocky mudstones with low-chroma, olive drab −1.8‰ to −24.4‰ and 7.8‰ to 28.8‰, respectively. to light greenish gray matrix, yellow and red mottles and The C and O isotope compositions of radiaxial calcite, mm to cm-scale iron and carbonate nodules. Calcisols microspar and micrite all overlap, and there appears to are massive to very weak medium angular blocky, light be no significant correlation between δ13C value and greenish gray to dusky red profiles with nodular (5 mm calcite texture, although radiaxial calcite δ13C values to 120 mm in diameter) and laminar (b150 mm thick) are slightly more negative than the other calcite carbonate beds. components (Fig. 4). However, significant differences exist between the δ13C values of carbonate nodules 4.1. Calcite cements from coeval beds from Sections 1 and 2 (Fig. 4). Section 2 exhibits a comparatively narrow range of negative Three distinct calcite textures occur in nodules: δ13C values,−21.9‰ to −16.8‰, whereas Section 1 radiaxial, microspar and micrite. Radiaxial calcite, the exhibits a more variable and generally more positive most common texture, occludes detrital grains and mm- range of δ13C values, −24.4‰ to −1.8‰. scale micritic clasts (Fig. 3a, d). Stellate structures ranging from ∼1 μm to 5 mm wide occur in sediments, 5. Discussion paleosol matrix, and calcareous nodules (Fig. 3b). Their growth habit indicates that they are pseudomorphs of An ∼−10‰ δ13C excursion in carbonate nodules gypsum similar to gypsum rosettes that occur in Carlton from several P–T stratigraphic sections across the Heights paleosols (Retallack et al., 2003). Calcite Karoo basin was observed by MacLeod et al. (2000) microspar occludes detrital grains (Fig. 3c). Micrite is and Ward et al. (2005). Both studies interpret this ne- less common and occurs primarily as aphanocrystalline gative δ13C excursion to record changing δ13C values mm-scale inclusions within radiaxial and microspar of atmospheric CO2 and to reflect the negative carbon

13 18 Fig. 4. Plot of δ CVPDB vs. δ OVSMOW values of calcite from nodules in the Carlton Heights sequence. See text for discussion. N.J. Tabor et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 370–381 377 isotope spike found in P–T marine sections around the the δ13C values of calcite from Sections 1 and 2; Sec- world. tion 2 δ13C values are 9‰ to 14‰ more negative than In order for the paleosol carbonate nodule δ13C penecontemporaneous Section 1 samples. excursion in the Karoo basin to be mechanistically Organic matter δ13C values from Carlton Heights linked and synchronous with the negative carbon iso- (Ward et al., 2005) range from −25.4‰ to −22.4‰, tope excursion associated with the marine P–T bound- which is a typical range for C3 photosynthesizers ary, those Karoo basin values must accurately reflect the (Fig. 5). Considering the aforementioned +14.8‰ composition of contemporaneous atmospheric carbon. fractionation between organic matter and calcite, the However, not all soil (and paleosol) calcites form by range of calcite δ13C values representative of equilib- open-system gaseous exchange with tropospheric car- rium with CO2 derived from oxidation of organic matter bon (e.g., Cerling, 1984, 1991). Thus, not all paleosol in Carlton Heights paleosol carbonates is −10.6‰ to carbonates provide faithful proxies of atmospheric δ13C −7.6‰ (Fig. 5). Note that C3 organic matter δ13C values (Cerling, 1992; Ekart et al., 1999; Deutz et al., values are ∼16‰ to 18‰ more negative than ambient 2001; Tabor et al., 2004). atmospheric CO2 (e.g., Ekart et al., 1999). Therefore, Cerling (1991) defined a two end-member mixing addition of any tropospheric CO2 into the soil system model that relates the isotopic composition of soil cal- during calcite crystallization could have only resulted in 13 13 cite to the concentration (and δ C) of atmospheric CO2 more positive calcite δ C values than those calculated for soils characterized by open-system, one-dimensional using the organic matter δ13C values alone (Fig. 5). In Fickian diffusive, gaseous transport. One of the primary this regard, Carlton Heights' organic matter δ13C values stipulations of this model is that soils must be well- effectively define a lower limit of permissible calcite drained, because carbonate in such profiles is more δ13C values for open system diffusive exchange with likely to have formed under open system conditions. paleo-soil CO2 and global atmospheric CO2. In other Soil calcite δ13C values in well-drained soils record words, calcite δ13C values more negative than −10.6‰ mixing of two end-member CO2 components: CO2 de- cannot plausibly offer any meaningful information 13 rived from in-situ oxidation of biological material in the about global atmospheric CO2 δ C values. Micrite 13 soil and CO2 from the troposphere (Cerling, 1984). δ C values in Section 1 range from −7.1‰ to −5.2‰, Calcite δ13C values originating solely from oxidation of which are more positive than the anticipated calcite biological material in the soil profile will be at least δ13C values calculated from contemporaneous organic 14.8‰ heavier than the oxidizing soil organic matter matter and therefore may reflect records of tropospheric under open-system conditions. This minimum isotopic δ13C values (Fig. 5). However, all of the micritic calcite difference between coexisting carbonate and organic samples from nodules in Section 2 have δ13C values matter reflects a 4.4‰ enrichment associated with more negative than the permissible lower limit (Fig. 5). 13 diffusive transport of biologically-derived CO2 (Cerling, Therefore, these more negative δ C values suggest 1991) and an additional 10.4‰ carbon isotope enrich- that nodule formation occurred (1) under soil conditions 13 ment from gaseous CO2 to calcite due to carbon isotope that yielded more negative calcite δ C values and/or fractionation between carbonate species (at mildly (2) not through open-system diffusion between soil- alkaline pH; Bottinga, 1968). derived and tropospheric CO2. Previous isotope studies of Karoo P–T carbonate It is possible that the organic matter δ13C values in nodules provide no systematic analysis of calcite texture. the Karoo sediments reflect diagenetic enrichment of Yet, a practical procedure for assuring measurement of organic matter δ13C values through selective oxidation paleosol calcite δ13C values that are derived solely from of starches and sugars that drove organic carbon toward well-drained conditions has been to eliminate microspar, more positive values by increasing the lignin fraction in radiating crystal morphologies and septarian calcite tex- the organic matter (e.g., Nadelhoffer and Fry, 1988; tures from consideration, because they are commonly Schweizer et al., 1999; Ehleringer et al., 2000; Bowen formed in groundwater environments or during burial and Beerling, 2004). The 1 to 2‰ enrichment that is diagenesis (Cerling, 1991, 1992; Ekart et al., 1999; Deutz anticipated due to organic matter degradation, however, et al., 2001). Rather, micrite is the most appropriate (and cannot account for the magnitude of the isotopic offset reliable) texture to ensure the greatest likelihood that between measured organic matter (Ward et al., 2005) pedogenic calcite preserves records of open system and calcite δ13C values at Carlton Heights (Fig. 5). 13 exchange with atmospheric CO2 (Cerling, 1991; Ekart Alternatively, carbonate nodule δ C values may re- et al., 1999). Micrite δ13C values from Carlton Heights cord diagenetic alteration. The combination of dissolu- are plotted in Fig. 5. A significant offset exists between tion structures in micritic textures (Fig. 3d), a regional 378 N.J. Tabor et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 370–381

Fig. 5. Plot of the stratigraphic position vs. δ13C value of micritic calcite and organic matter. The 0 m level in this figure was arbitrarily chosen at the base of section 1. Open and closed diamonds represent micrites, presented in this work for the first time, from sections 1 and 2, respectively, whereas crosses depict organic matter values at Carlton Heights from Ward et al. (2005). Open circles are calculated δ13C values of calcite if they form in equilibrium with only CO2 derived from oxidation of organic matter in a well-drained soil characterized by open system Fickian diffusive transport of 13 CO2. Thin, solid black and gray lines depict the approximate permissible range of calcite δ C values that could represent equilibrium with CO2 derived from oxidation of Carlton Heights' organic matter. Thin dotted line is negative limit of calculated calcite δ13C values assuming that bulk organic matter underwent 2‰ diagenetic enrichment. Black dashed horizontal lines mark positions of the perceived P–T boundaries at Carlton Heights proposed in other studies (MacLeod et al., 2000, Retallack et al., 2003; Steiner et al., 2003; Ward et al., 2005). Thick solid black and gray bars depict the range of bulk paleosol carbonate nodule δ13C values in P–T strata of the Carlton Heights, Lootsberg Pass and Bethulie sections published in MacLeod et al. (2000) and Ward et al. (2005), respectively. These bulk paleosol calcite nodule δ13C values could not be put into stratigraphic order with the Carlton Heights section presented in this work, because that information is not available. Nevertheless, bulk paleosol carbonate data from MacLeod et al. (2000) and Ward et al. (2005) are penecontemporaneous with the data presented here, and any carbonate nodule δ13C valuesb

−10.6‰ from those studies did not likely form in the presence of global tropospheric CO2. See text for further discussion.

Jurassic intrusive igneous event (Hargraves et al., 1996), nificantly changed from their Permo–Triassic values. and strikingly negative δ18O values of calcite cements Although additional possibilities may also exist, the (Fig. 4) are suggestive of diagenetic modification. Nev- very negative calcite δ13C values may reflect the ertheless, because diagenetic waters are typically car- primary depositional environment. bon-poor, with very little chemical potential to facilitate Notably, the morphologies of the paleosols indicate carbon isotope change (Tucker and Wright, 1990), δ13C that few of the paleosol profiles at Carlton Heights of the micritic calcite values are not likely to be sig- appear to have formed under very well-drained conditions N.J. Tabor et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 370–381 379

(Smith, 1995; Retallack et al., 2003). The lack of a well- Moreover, gypsum commonly forms in poorly developed pedogenic structure in the paleosols suggests drained, swampy soils (e.g., Aslan and Autin, 1998); this interpretation, as does the lack of horizonation and the calcite pseudomorphs of gypsum rosettes at the soil reddening, which is typical of older, stable, and very Carlton Heights section suggest there was an available well drained sites of modern landscapes. In fact, Carlton source of both sulfate for oxidation of organic matter Heights profiles exhibit abundant redoximorphic fea- and Ca2+ for calcite crystallization under closed system tures such as drab gray, light yellow and green mottles in conditions. The very negative δ13C values of calcite and a dark red paleosol matrix or concentrations of red, the occurrence of gypsum rosette pseudomorphs re- orange and bright yellow mottles or nodules (e.g., ported in the P–T boundary strata of the Karoo basin hematite, goethite, jarosite; cf. Steiner et al., 2003)ina (Retallack et al., 2003) suggest that closed system drab paleosol matrix that are indicative of poorly drained oxidation of organic matter likely generated the 13C- and protracted saturation in hydromorphic soils (Duch- depleted calcite reported here and in previous studies aufour, 1982; Soil Survey Staff, 1998). By analogy to (MacLeod et al., 2000; Ward et al., 2005). Nevertheless, modern soils, paleosol profiles at Carlton Heights likely 13C-depleted calcite occupies a very small rock volume record pedogenesis in seasonally to continuously in the Karoo P–T strata, probably b0.1%. If all of the saturated, poorly drained and swampy landscapes carbonate in these sedimentary strata were derived from situated near (or beneath) the local groundwater table. in-situ oxidation of organic matter by sulfate reduction Poorly drained and waterlogged soils tend to develop of gypsum, it would have required no less than 27 mol as chemically closed (or semi-closed, but hereafter of sulfate reduction per m3 of sediment with a C3 simply referred to as closed) systems, where gaseous organic matter content of 0.8 kg/m3 (∼0.03 wt.% as diffusion between the troposphere and soil is absent CH2O). Wetlands are dominated by C3 vegetation, and or severely limited. Under these conditions, aqueous O2 the concentration of organic matter in modern bottom- is quickly consumed by oxidation of organic matter lands is 80 times greater than the amount needed to and the soil (and sediment) becomes anoxic (Feng and generate the calcite that is present in these P–T strata Hsieh, 1998). Nevertheless, oxidized carbon species (Schlesinger, 1997). Wetlands are net retainers of − 2− (CO2 (aq), HCO3 ,CO3 ) may be liberated from organic sulfate, with pore-water concentrations as high as matter in the absence of O2 by bacterial sulfate re- 250 mg/L (e.g., Warren et al., 2001). Based on these duction, thus maintaining solution pH permissive of maximum sulfate concentrations in modern environ- calcite precipitation (Irwin et al., 1977): ments, ∼10 m3 of swamp water could have provided 3 − − the minimum amount of gypsum (within each m of SO2 þ 2CH O→H S þ 2HCO 4 2 2 3 sediment) necessary to produce the 13C-poor calcite Bicarbonate and calcite will have δ13C values that preserved in the P–T strata at Carlton Heights. This are equivalent to that of the oxidizing organic matter in represents a relatively small water/rock ratio, even in such systems, because all of the carbon is transferred poorly drained swamplands. Sulfur isotope geochemis- from one phase to another without fractionation (Irwin try and S/C ratios across the P–T boundary in the et al., 1977; Feng and Hsieh, 1998). northern Karoo basin suggest that freshwater sulfate Whelan and Roberts (1973) cored and measured δ13C concentrations were very high (Maruoka et al., 2003) values of calcite nodules from the Atchafalaya Swamp as a result of atmospheric SO4 deposition (possibly) near Port Barre, Louisiana. Calcite δ13C values from a associated with Siberian trap volcanism. If true, the well-drained site exhibited minimal carbon isotope vari- probability for 13C-depleted calcite crystallization in ation and ranged from −11.3 to −12.6‰. The poorly poorly drained bottomlands would have been enhanced drained, waterlogged site exhibited a much broader and during the P–T transition. more negative δ13C range, from −10.6‰ to −20.2‰. Such negative δ13C calcite values in the poorly drained 6. Conclusions profile are attributed to crystallization of carbonate from oxidation of C3 organic matter under closed system Paleosol morphological indicators suggest that P–T conditions. Therefore, the physico–chemical properties boundary strata were deposited in poorly drained, that cause crystallization of 13C-poor calcite are found in swampy soils across much of the Karoo basin. Calcite poorly drained, freshwater environments similar to those nodules associated with paleosol profiles exhibit radia- paleoenvironmental conditions suggested by paleosol xial, microspar and micritic textures. Only the micritic morphologies (Smith, 1995; Retallack et al., 2003) for calcite nodules are appropriate for development of a the latest Permian Karoo basin. δ13C chemostratigraphy for correlation to marine δ13C 380 N.J. Tabor et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 370–381 records. Furthermore, calcite δ13C values more positive Cerling, T.E., 1991. Carbon dioxide in the atmosphere: evidence from than −7.6‰, such as those measured in Section 1 at Cenozoic and Mesozoic paleosols. American Journal of Science 291, 377–400. Carlton Heights, likely record crystallization from Cerling, T.E., 1992. Use of carbon isotopes in paleosols as an indicator carbonate species in oxygen-rich waters within better- of the PCO2 of the paleoatmosphere. Global Biogeochemical drained areas of the paleolandscape (e.g. Whelan and Cycles 6, 307–314. Roberts, 1973). However, calcite δ13C values more Craig, H., 1957. Isotopic standards for carbon and oxygen and negative than −10.6‰, such as those measured in correction factors for mass spectrometric analysis of carbon dioxide. Geochimica et Cosmochimica Acta 12, 133–149. Section 2 at Carlton Heights and reported as evidence Deutz, P., Montanez, I.P., Monger, H.C., Morrison, J., 2001. Morphology for the P–T boundary from other sections in the Karoo and isotope heterogeneity of Late Quaternary pedogenic carbonates: basin (MacLeod et al., 2000; Ward et al., 2005), do not implications for paleosol carbonates as paleoenvironmental proxies. Palaeogeography, Palaeoclimatology, Palaeoecology 166, 293–317. reflect the isotopic composition of atmospheric CO2, and therefore do not provide a viable chemostratigraphic de Wit, M.J., Ghosh, J.G., de Villiers, S., Rakotosolofo, N., Alexander, J., Tripathi, A., Looy, C., 2002. Multiple organic carbon isotope correlation with the global negative carbon isotope reversals across the Permo–Triassic boundary of terrestrial excursion and mass extinction near the P–T boundary. Gondwana sequences; clues to extinction patterns and delayed 13 These extremely negative δ C values more likely ecosystem recovery. Journal of Geology 110, 227–246. reflect calcite precipitation derived from oxidation of Duchaufour, P., 1982. Pedology: Pedogenesis and Classification. organic matter in anoxic environments, possibly through George Allen and Unwin, London. 448 pp. Ehleringer, J.R., Buchmann, N., Flannagan, L.B., 2000. Carbon sulfate reduction, in extremely poorly drained parts isotope ratios in belowground carbon cycle processes. Ecological of the paleolandscape. Although seemingly peculiar, Applications 10, 412–422. the circumstances proposed here for the origin of Karoo Ekart, D.D., Cerling, T.E., Montanez, I.P., Tabor, N.J., 1999. A P–T paleosol calcite nodules require no special or 400 million year carbon isotope record of pedogenic carbonate: unique conditions atypical of modern wetlands and implications for paleoatmospheric carbon dioxide. American Journal of Science 299, 805–827. swamps (Whelan and Roberts, 1973; Feng and Hsieh, Feng, J., Hsieh, Y.P., 1998. wetlands and aquatic processes; sulfate 1998). However, because of these particular circum- reduction in freshwater wetland soils and the effects of sulfate and stances the Karoo P–T paleosol carbonate nodules substrate loading. Journal of Environmental Quality 27, 968–972. should not be used for chemostratigraphic correlation to Gastaldo, R.A., Adendorff, R., Bamford, M.K., Labandeira, Neveling, extrabasinal sites. J., Sims, H.J., 2005. Taphonomic trends of macrofloral assemblages across the Permian–Triassic boundary, Karoo Basin, South Africa. Palaios 20, 478–497. Acknowledgements Gonfiantini, R., 1984. Advisory group meeting on stable isotope reference samples for geochemical and hydrological investiga- This research was funded by the National Science tions. Representative for the director general of the International Foundation grants EAR-0447381 to N.J. Tabor and Atomic Energy Association, Vienna. Groenewald, G.H., 1989. Stratigrafie en sedimentologie van die Groep EAR-0545654 to N.J. Tabor and I.P. Montañez. 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