Sulfur Geochemistry Across a Terrestrial Permian^Triassic Boundary Section in the Karoo Basin, South Africa

Earth and Planetary Science Letters 206 (2003) 101^117 www.elsevier.com/locate/epsl

Sulfur geochemistry across a terrestrial Permian^Triassic boundary section in the Karoo Basin, South Africa

T. Maruoka a;, C. Koeberl a, P.J. Hancox b, W.U. Reimold b

a Institute of Geochemistry, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria b Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa

Received 1 July 2002; received in revised form 8 October 2002; accepted 19 November 2002

Abstract

Concentrations of sulfur and carbon, and isotopic compositions of sulfur were determined in sedimentary rocks from a section across the terrestrial Permian^Triassic (P^Tr) boundary in the northern Karoo Basin, South Africa. High concentrations of sulfide produced by sulfate-reducing bacteria were found in the sedimentary rocks at and just below the perceived P^Tr boundary. The ratios of organic carbon to sulfide of the sedimentary rocks with high concentrations of sulfide are relatively constant and similar to those of marine environments. This means that the enhanced accumulation of sulfide is a result of the enrichment of sulfate in the water. As the sulfide concentrations do not correlate with concentrations of any major elements indicative of weathering intensity, the supply of sulfate was independent of environmental factors, such as temperature and CO2 concentrations in the atmosphere, which control weathering intensity. Therefore, it is reasonable to consider that the sulfate might have been supplied to the freshwater directly as acid rain by an event such as a bolide impact or a volcanic eruption. The absence of evidence for an impact event in the geological record, though, favors volcanic input as the cause of the acid rain. ß 2002 Elsevier Science B.V. All rights reserved.

Keywords: terrestrial Permian^Triassic boundary section; Siberian £ood volcanism; sul¢de accumulation; sulfur isotopic composition

1. Introduction (e.g. [1^3]). To date, the cause of this extinction remains unsolved, although many di¡erent mech- The mass extinction at the Permian^Triassic anisms have been proposed (see [4]), including a (P^Tr) boundary has been recognized as the bolide impact (e.g. [5,6]), rapid, global sea-level most signi¢cant extinction in the history of life rise (e.g. [7,8]), oceanic anoxia (e.g. [9,10]), and volcanism (e.g. [11,12]). Recent suggestions regarding the presence of extraterrestrial fullerenes in sedimentary rocks at * Corresponding author. Present address: Laboratory for the P^Tr boundary in China [13] remain contro- Space Sciences, Physics Department, Washington University, Campus Box 1105, 1 Brookings Drive, St. Louis, MO 63130- versial [14^16], but the bolide impact theory for 4899, USA. Tel.: +1-314-935-6206; Fax: +1-314-935-4083. the P^Tr mass extinction has also been favored by E-mail address: [email protected] (T. Maruoka). others (e.g. [5,6,17]), especially as recent work has

EPSL 6496 8-1-03 Cyaan Magenta Geel Zwart 102 T. Maruoka et al. / Earth and Planetary Science Letters 206 (2003) 101^117 shown that this mass extinction occurred within a hanced accumulation of sul¢de in freshwater sedi- relatively short period of time (e.g. [2,5,18^22]). ments could also be induced in a highly reduced Kaiho et al. [17] reported a negative 34S/32S environment; however, the enhanced sul¢de accu- excursion for marine sulfate at the end of the mulation under highly reduced conditions can be Permian, accompanied by a negative 87Sr/86Sr ex- distinguished from that in a highly sulfate-rich cursion that has been observed at many locations environment, based on the ratio of organic car- (e.g. [23,24]). As both isotope excursions can be bon to sul¢de in the sediments [32^34]. The ratio induced by an addition of mantle material to sea- of organic carbon to sul¢de produced in a very water, Kaiho et al. [17] proposed that a large sulfate-rich environment is relatively constant and asteroid or comet impact caused rapid and mas- lower than that for freshwater with normal sulfate sive release of sulfur from the mantle to the concentrations, because sul¢de accumulations in ocean^atmosphere system, leading to oxygen con- highly sulfate-rich water may be limited by organ- sumption, acid rain, and, consequently, a severe ic input [32^34]. biotic crisis. However, this proposal is highly con- Here, we present concentrations of sulfur and troversial, as their interpretation requires the im- carbon, as well as N34S values, for a suite of sedi- pact of an asteroid similar in size to the largest mentary rocks from a section straddling the ter- known asteroid in the solar system, and there is restrial P^Tr boundary at Senekal in the northern no physical evidence for such large impacts exca- Karoo Basin of South Africa (Fig. 1). We discuss vating the mantle and causing massive outgassing the implications of these data for understanding at the P^Tr boundary; also, the chemical evidence the possibly changing paleo-environmental condi- cited is badly documented and ambiguous ([16] tions across the P^Tr boundary. provides a more detailed discussion). If a gigantic amount of sulfur had been supplied to the surface of the Earth by a process 2. Samples like that advocated by Kaiho et al. [17] or by another process, such as a volcanic SO2 eruption The £uvially deposited sedimentary rocks of the or a comet or asteroid impact [25,26], the supplied Beaufort Group of the Karoo Supergroup cover a sulfur should have a¡ected the sedimentation of time span ranging from the Middle Permian to sulfur in freshwater, as well as in marine environ- the Middle Triassic [35,36] and include a complete ments. As concentrations of sulfate in freshwater and well-studied non-marine P^Tr boundary se- are generally lower than those in oceanic water quence (e.g. [22,37,38]). Two sections, designated (e.g. world average of river water, 120 WM [27]; as A and B by [38], across the P^Tr boundary, recent seawater, 28 900 WM [27]), freshwater envi- from the east of the town of Senekal (Free State ronments are highly suitable for the detection of a Province; Fig. 2), respectively, were sampled for sudden enhanced input of sulfur. The accumula- this study. The two sections consist of horizon- tion of sul¢de produced by sulfate-reducing bac- tally laminated to massive, interbedded mudrock teria in modern lake sediments actually increases and siltstone, and ¢ne sandstone (Balfour Forma- as sulfate concentrations increase, due to acid pre- tion), overlain by a laterally continuous, horizon- cipitation as observed presently in highly polluted tally and trough cross-strati¢ed sandstone (Kat- areas (e.g. [28^30]). Maruoka et al. [31] inter- berg Formation) (Figs. 3 and 4). The Katberg preted the enrichment of sul¢de in the sedimenta- Formation sandstones display an erosive lower ry rocks at the terrestrial K^T boundary as the contact and, in places, have scoured away a sig- result of acid rain after the Chicxulub impact ni¢cant portion of the original clay surface. The event. Sul¢de concentrations in terrestrial sedi- samples were carefully collected from 1^2 cm wide mentary rocks could thus provide a tool to exam- segments in these sections. Detailed stratigraphic ine whether increase of sulfur input by unusual and mineralogic data on these sections, as well as processes, such as bolide impact or intense vol- the results of chemical and paleobiological analy- canic activity, might have occurred or not. En- ses of the samples utilized in the present study, are

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Fig. 1. Geographic distribution of the rocks of the main Karoo Basin, South Africa, showing the position of the town of Senekal in the northern part of the basin (after [74]). given by Hancox et al. [38]. Sample numbers (e.g. sulfur were measured in bulk samples as well as A31.0, B+2.0) correspond to the distance (in cm) in 2 M HCl residues, for some samples, using of a given sample from the boundary depth as helium-gas continuous-£ow isotope-ratio mass given by [38]. These authors concluded that the spectrometry (CF-IR-MS; Micromass Optima P^Tr boundary occurs between the Normandien [31]). The concentrations of carbon were mea- (northern Balfour equivalent) and Verkykerskop sured using a thermal conductivity detector (northern Katberg equivalent) formations, based (TCD; Carlo Erba) with a combustion furnace, on a change from Late Permian faunal commun- followed by mass spectrometric analysis. The ities, including Dicynodon, to a Triassic one dom- samples were weighed into 12U5 mm tin capsules, inated by a Lystrosaurus assemblage zone fauna together with a mixture of V2O5 and SiO2 to pro- (Fig. 3). Following the current terminology, we mote full combustion [39]. The sulfur isotopic here use Balfour and Katberg formations for the compositions are expressed in terms of N34S two sequences under discussion. The boundary (x) relative to the Canyon Diablo standard. presumed from the biostratigraphy is coincident Results of three IAEA silver sul¢de stand- with a boundary location derived chemostrati- ards (IAEA-S-1, 30.3x; IAEA-S-2, 22.67x; graphically [38]. IAEA-S-3, 332.55x [40]) were compared to constrain the N34S values. The isotopic compositions of sulfur were determined at a precision of 3. Experimental methods þ 0.2x (1c). The reference sul¢des were also used for the calibration of measured sulfur con- Concentrations and isotopic compositions of tents, determined at a precision of 3 rel%. A stan-

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dard (Acetanilide Standard, ThermoQuest Italia) was used for the calibration of carbon contents that were measured with a precision of V1 rel% for 2 mg C and V10 rel% for 0.2 mg C. Detec- tion limits for S and C concentrations for our procedures were about 10 ppm and 100 ppm, respectively.

4. Results

Sulfur and carbon concentrations, as well as sulfur isotopic compositions, of bulk samples and of 2 M HCl residues are given in Table 1. We could not determine the N34S values and carbon concentrations of the Permian samples of series A of [38], because their C and 34S concentrations were below the detection limits. These Fig. 2. Map of the town of Senekal showing the position of samples of series A might represent sedimentation the sections sampled for this work. in highly oxidized bottom waters. In well-oxygen-

Fig. 3. Generalized stratigraphic pro¢le of the P^Tr boundary at Senekal. Facies codes are as follows: Se, erosional scour ¢ll; Sei, intraformational scour ¢ll; Sh, horizontally strati¢ed sandstone; St, trough cross-strati¢ed sandstone; Stl, large-scale trough cross-strati¢ed sandstone; Sts small-scale trough cross-strati¢ed sandstone; Sr, ripple-strati¢ed sandstone; Sp, planar cross-strati- ¢ed sandstone; Fh, horizontally laminated ¢nes (mudrocks and siltstones); Fr, ripple-laminated ¢nes. Inset shows detailed stratigraphy of the sampled interval.

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Fig. 4. Photograph showing the P^Tr contact at Senekal. ated water, terminal electron-accepting processes, HCl treatment (e.g. [41,42]). Therefore, the devia- such as sulfate reduction, might not occur, or de- tion of the data for B39.0 from the line with the ¢ciency of organic matter might prohibit the bac- slope of 1 could also be explained by e¡ects re- terial reduction of sulfate. lated to acid dissolution of labile organic matter, and it is reasonable to consider the concentrations 4.1. Comparison of carbon abundances for bulk of total carbon in the bulk samples as those of samples and their 2 M HCl residues purely organic carbon.

Fig. 5 shows a comparison of carbon and sulfur 4.2. Comparison of sulfur abundances and N 34S concentrations, and N34S values for bulk sedimen- values for bulk samples and their 2 M HCl tary rocks and 2 M HCl residues. In Fig. 5a,b, the residues concentrations of carbon and sulfur in the residues (Cresidue) were converted to those of acid-in- All sulfur concentrations for 2 M HCl residues, soluble carbon and sulfur in the bulk samples except for sample B+4.4-5.0, are lower than those (Cinsoluble in bulk), according to for the respective bulk samples (Fig. 5b). This indicates that bulk samples include acid-soluble Cinsoluble in bulk ¼ Cresidue as well as acid-insoluble sulfur. As seen in Fig. 5b, the ratios of acid-insoluble sulfur to total sul- Uðresidue weightÞ=ðoriginal weightÞ fur are variable. This agrees with ratios of acid- soluble sul¢de to acid-insoluble sul¢de for actual All carbon concentrations, except that for sample lake sediments ranging from 0.01 to 100 [43]. The B39.0, fall onto a line with a slope of 1 (solid line acid-soluble sul¢de cannot be detected by com- in Fig. 5a), within error limits, con¢rming that the paring total and acid-insoluble sulfur if the ratios carbon in the bulk samples consists only of acid- of acid-soluble sul¢de to acid-insoluble sul¢de are insoluble, i.e. organic, carbon, consistent with small. X-ray di¡raction results of [38]. Although concen- The N34S values of the acid residues are equal to trations of total carbon appear to be slightly high- or slightly lower than those of the corresponding er than those of organic carbon determined from bulk samples (Fig. 5c). As N34S values of acid- the acid residues (Cinsoluble/Ctotal = 0.84 þ 0.09; dot- soluble sul¢de (monosul¢de) are generally higher ted line in Fig. 5a), this could be the result of than those of acid-insoluble sul¢de (disul¢de) in partial loss of the organic fraction during 2 M freshwater sediments (e.g. [44,45]), the small dif-

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Table 1 Concentrations of carbon and sulfur and isotopic compositions of sulfur in P^Tr sedimentary rocks

Sample Distance Bulk analysis 2 M HCl residue from PTBa

CS N34S C/S C S N34S (cm) (wt%) (wt%) (x) (wt%) (wt%) (x)

A+8.5 8.5 0.3118 þ 0.1028 1.0129 þ 0.2271 34.25 þ 0.12 0.31 þ 0.12 0.4272 þ 0.1240 1.0601 þ 0.1683 33.84 þ 1.08 A+3.5 3.5 0.0315 þ 0.0047 0.0233 þ 0.0043 34.27 þ 1.50 1.35 þ 0.32 0.0259 þ 0.0033 0.0157 þ 0.0033 36.28 þ 0.19 A+1.5 1.5 1.3266 þ 0.5356 3.1957 þ 0.8833 38.29 þ 0.84 0.42 þ 0.20 1.2240 þ 0.3981 2.7599 þ 1.1726 310.00 þ 1.54 A30.9 30.9 0.0058 þ 0.0003 A31.0 31 0.0059 þ 0.0007 A31.5 31.5 0.0037 þ 0.0001 A32.0 32 0.0055 þ 0.0003 A33.0 33 0.0071 þ 0.0010 A35.0 35 0.0060 þ 0.0002 A35-7 36 0.0032 þ 0.0004 A38.0 38 0.0056 þ 0.0003 A311.0 311 0.0050 þ 0.0002 A313.0 313 0.0049 þ 0.0013

B+15.0 15 0.0519 þ 0.0045 0.0356 þ 0.0011 30.18 þ 0.28 1.46 þ 0.14 B+12.0-15.0 13.5 0.0354 þ 0.0031 0.0232 þ 0.0014 30.97 þ 0.67 1.53 þ 0.16 0.0297 þ 0.0007 0.0054 þ 0.0001 30.30 þ 0.21 B+12.5 12.5 0.0459 þ 0.0059 0.0451 þ 0.0035 31.18 þ 0.24 1.02 þ 0.15 B+11.5 11.5 0.0622 þ 0.0029 0.0151 þ 0.0020 2.88 þ 0.59 4.13 þ 0.59 B+10.0 10 0.0516 þ 0.0090 0.0055 þ 0.0008 2.12 þ 1.29 9.42 þ 2.16 B+8.0 8 0.0358 þ 0.0012 0.0046 þ 0.0007 3.58 7.82 þ 1.27 B+6.5 6.5 0.0329 þ 0.0003 0.0040 þ 0.0011 2.84 8.28 þ 2.22 B+4.4-5.0 4.7 0.0786 þ 0.0016 0.0175 þ 0.0009 311.00 þ 0.31 4.49 þ 0.26 0.0691 þ 0.0017 0.0180 þ 0.0014 311.86 þ 0.46 B+4.0 4 0.1018 þ 0.0025 0.0046 þ 0.0003 1.21 þ 0.69 22.02 þ 1.38 B+3.0 3 0.0471 þ 0.0037 0.0081 þ 0.0004 0.27 þ 0.54 5.79 þ 0.53 0.0406 þ 0.0038 0.0053 þ 0.0003 0.44 þ 0.22 B+2.0 2 0.0594 þ 0.0049 0.0054 þ 0.0003 1.24 þ 0.32 11.05 þ 1.13 B+1.0 1 0.0635 þ 0.0047 0.0197 þ 0.0013 32.72 þ 0.78 3.23 þ 0.32 0.0560 þ 0.0006 0.0124 þ 0.0008 34.44 þ 0.40 B3OA 0 0.1054 þ 0.0039 0.0470 þ 0.0011 1.95 þ 0.57 2.24 þ 0.10 0.0933 þ 0.0109 0.0214 þ 0.0008 1.89 þ 0.18 B3OB 0 0.0907 þ 0.0080 0.0263 þ 0.0031 1.63 þ 0.91 3.44 þ 0.51 B3OC 0 0.1442 þ 0.0026 0.0463 þ 0.0059 0.26 þ 0.33 3.11 þ 0.40 B30.5A 30.5 0.0950 þ 0.0105 0.0269 þ 0.0019 37.29 þ 0.86 3.53 þ 0.46 0.0824 þ 0.0162 0.0228 þ 0.0015 39.36 þ 0.50 B30.5B 30.5 0.3123 þ 0.0584 0.1535 þ 0.0083 32.69 þ 0.29 2.03 þ 0.40 B30.5C 30.5 0.0813 þ 0.0114 0.0569 þ 0.0057 311.44 þ 0.31 1.43 þ 0.25 0.0722 þ 0.0091 0.0497 þ 0.0028 312.66 þ 0.77 B31.0 31 0.1415 þ 0.0216 0.1041 þ 0.0185 2.16 þ 0.44 1.36 þ 0.32 0.1272 þ 0.0274 0.0519 þ 0.0090 2.80 þ 0.29 B31.5 31.5 0.1020 þ 0.0116 0.0413 þ 0.0026 3.25 þ 0.31 2.47 þ 0.32 B32.5 32.5 0.1044 þ 0.0085 0.0595 þ 0.0062 2.37 þ 0.35 1.76 þ 0.23 B33.5 33.5 0.1324 þ 0.0281 0.1061 þ 0.0138 2.11 þ 0.66 1.25 þ 0.31 B34.0 34 0.1549 þ 0.0105 0.0810 þ 0.0038 2.40 þ 0.38 1.91 þ 0.16 B35.0A 35 0.1176 þ 0.0155 0.1330 þ 0.0047 2.34 þ 0.62 0.88 þ 0.12 B35.0B 35 0.0735 þ 0.0031 0.0625 þ 0.0016 2.03 þ 0.35 1.18 þ 0.06 0.0646 þ 0.0053 0.0333 þ 0.0023 2.21 þ 0.34 B38.5 38.5 0.0636 þ 0.0005 0.0195 þ 0.0030 2.41 þ 0.58 3.26 þ 0.50 0.0575 þ 0.0047 0.0083 þ 0.0003 3.87 þ 0.78 B39.0 39 0.1055 þ 0.0056 0.0554 þ 0.0036 2.28 þ 0.18 1.90 þ 0.16 0.0739 þ 0.0011 0.0201 þ 0.0022 2.49 þ 0.47 B310.0 310 0.0404 þ 0.0007 0.0023 þ 0.0002 2.23 17.31 þ 1.31 B311.0 311 0.0456 þ 0.0020 0.0111 þ 0.0011 3.98 þ 0.17 4.11 þ 0.46 B312.0 312 0.0537 þ 0.0093 0.0153 þ 0.0033 2.86 þ 0.58 3.51 þ 0.97 0.0386 þ 0.0014 0.0051 þ 0.0002 2.35 þ 0.75 B313.0 313 0.0616 þ 0.0068 0.0380 þ 0.0056 0.61 þ 0.33 1.62 þ 0.30 B314.0 314 0.0497 þ 0.0013 0.0175 þ 0.0034 0.19 þ 0.64 2.85 þ 0.55 B316.0 316 0.0503 þ 0.0060 0.0206 þ 0.0027 12.92 þ 0.84 2.45 þ 0.44 0.0534 þ 0.0024 0.0108 þ 0.0005 13.51 þ 0.53 B317.5 317.5 0.0455 þ 0.0024 0.0176 þ 0.0010 1.85 þ 0.39 2.58 þ 0.20 B318.0 318 0.0341 þ 0.0023 0.0044 þ 0.0003 2.73 þ 1.12 7.74 þ 0.71 B320.0 320 0.0303 þ 0.0014 0.0045 þ 0.0005 4.39 þ 1.91 6.75 þ 0.83 B325.0 325 0.0262 þ 0.0034 0.0016 þ 0.0003 5.28 þ 0.55 16.12 þ 3.50 B337.0 337 0.0363 þ 0.0078 0.0132 þ 0.0014 0.65 þ 0.33 2.75 þ 0.67 Errors are 1c for C and S concentrations and N34S. a PTB = Permian^Triassic boundary. ferences between the N34S values for acid residue sible acid-soluble sulfur components: sulfate pro- and bulk sample can be explained by partial loss duced by recent weathering [46] and sulfate that during 2 M HCl treatment as acid-soluble sul¢de. survived after incomplete reduction. As the for- Two types of sulfate must be considered as pos- mer sulfate ought to be depleted in the 34S relative

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Fig. 6. Di¡erences between the N34S values of total sulfur and acid-insoluble sulfur against the ratio of concentrations of acid-insoluble sulfur to total sulfur. Open and gray circles represent data for the A and B series, respectively.

to the original sul¢de [47], the contribution of this sulfate to the acid-soluble sulfur of our samples is minimal. The latter sulfate that survived after incomplete reduction should be enriched in 34S relative to the corresponding sul¢de (e.g. [48]). As- suming that the acid-soluble sulfur consisted mainly of sulfate escaped from bacterial reduction, the di¡erence of N34S values for total sulfur and (insoluble) sul¢de should depend on the abundance ratio of sul¢de to total sulfur. This di¡erence should decrease when approaching complete reduction, which should correspond to

6 Fig. 5. Abundances of (a) carbon and (b) sulfur, as well as (c) isotopic compositions of sulfur, for bulk samples against respective values for 2 M HCl residues from the same samples, from sections across the P^Tr boundary in the northern Karoo Basin. Open and gray circles represent data for samples from the A and B series, respectively. Concentrations of carbon and sulfur of A+1.5 and A+8.5 fall outside of the concentration ranges shown in panels a and b. Solid lines represent 1:1 relationships. Dotted line in panel a represents a regression line based on the data for samples of series B. Isotopic compositions are given as deviation from that of Canyon Diablo Troilite (CDT).

EPSL 6496 8-1-03 Cyaan Magenta Geel Zwart 108 T. Maruoka et al. / Earth and Planetary Science Letters 206 (2003) 101^117 a high ratio of sul¢de and total sulfur. This is, position for the maximum sul¢de concentrations however, not the case for our results. As seen in (Fig. 7d). Fig. 6, smaller di¡erences between acid-insoluble and total sulfur can be observed for lower Sresidue/ Stotal ratios. Therefore, it can be concluded that 5. Discussion the contribution of sulfate escaped from sulfate reduction to the acid-soluble sulfur is minimal. 5.1. C/S minimum below the P^Tr boundary These ¢ndings suggest that it is reasonable to re- gard the residual and dissolved sulfur, after the The C/S ratios for the series B samples show acid treatment, as acid-insoluble sul¢de (pyrite; two minima, namely one between 5.0 and 0.5 cm FeS2) and acid-soluble sul¢de (monosul¢de), re- below the P^Tr boundary, and the other at 12^15 spectively. cm above the boundary (Fig. 7d). These ratios are similar to C/S ratios for marine sediments 4.3. Chemostratigraphic pro¢les of sulfur and (1.8 þ 0.5 for Devonian to Tertiary marine shale carbon abundances [32]). Sul¢de-accumulation rates in the oceans are limited by input of organic material due to de¢- The concentrations of carbon and sulfur, and ciency of organic carbon; thus, C/S ratios of ma- the N34S values, all show maxima or minima at the rine sediments are relatively constant [33,34]. Sul- same stratigraphic position (Figs. 7a^c), whereas ¢de accumulations in freshwater environments are the C/S ratio shows a minimum just below the not generally limited by organic input, because

20 (a) (b) (c) (d)

-10

-20

-30 Distance from P-T boundary (cm) boundary from P-T Distance

-40 00.10.20.300.050.10.15 -15 -10 -5 0 5 10 15 01020 C (wt%) S (wt%) δ34S (‰) C/S

Fig. 7. Chemostratigraphic pro¢les for (a) carbon abundance, (b) sulfur abundance, (c) N34S values for bulk samples, and (d) ratio of carbon to sulfur abundances, for sample traverses across the P^Tr boundary, as located by Hancox et al. [38]. Isotopic compositions are given as deviation from that of Canyon Diablo Troilite (CDT).

EPSL 6496 8-1-03 Cyaan Magenta Geel Zwart T. Maruoka et al. / Earth and Planetary Science Letters 206 (2003) 101^117 109 availability of organics is higher and sulfate concentrations are lower in freshwater environments than in marine environments. Ratios of concen- 10 B-16.0 tration of organic carbon to sul¢de for freshwater sediments are variable and generally higher than

those of marine sediments [32^34]. The C/S ratios )

of freshwater sediments are similar to those of ‰ ( marine sediments, which implies that either the 0 S

organic concentrations were relatively lower, or 34 that the sulfate concentrations were much higher δ B+12.0>15.0 than those of normal freshwater environments. As the low C/S ratios of the lower interval (5^0.5 cm below the boundary) are accompanied by high -10 abundances of organic carbon, the marine C/S ratios observed in the samples below the P^Tr boundary should re£ect an environment for which 00.20.40.60.81 sulfate concentration was much higher than in normal freshwater environments. Thus, high sul- Acid-insoluble sulfide/Total sulfide fate was introduced into the freshwater related to Fig. 8. N34S values for acid-insoluble sulfur, which is inter- our sample suites at the time characterized by the preted to comprise mostly pyrite, plotted against the ratios marine C/S ratio. The cause of high sulfate supply of abundances of acid-insoluble sulfur to total sulfur. Iso- is discussed below (Section 5.3). topic compositions are given as deviation from that of Can- yon Diablo Troilite (CDT). Gray and open circles repre- N 34 sent data for series A and B samples, respectively. Solid line 5.2. Negative S excursion at the P^Tr boundary represents a linear regression line (R2 = 0.94) based on all the data for the B series, except for samples B316.0 and The N34S values of sul¢de are relatively constant B+12.0-15.0 with N34S value of +12.92 þ 0.84x and from 9 to 1 cm below the P^Tr boundary; how- 30.97 þ 0.67x, relative to CDT, respectively. ever, those at the boundary are variable and lower than the otherwise near-constant value (Fig. 7c). ¢de can be controlled by two parameters: redox The N34S values for sul¢de, except those for sam- state and pHof pore water in sediments. Mono- ples B316.0 and B+12.0-15.0, are well correlated sul¢de is formed through the reaction between Fe with the ratios of acid-insoluble sul¢de to total ions and H2S produced by sulfate-reducing bac- sul¢de (Fig. 8; R2 = 0.94 for samples of series teria. Disul¢de (pyrite) should be produced by B). As the Sinsoluble/Stotal ratio is controlled by en- reactions between monosul¢de and zero-valent vironmental parameters, such as redox state and sulfur species (elemental sulfur or polysul¢de 34 pH, this correlation means that the N S value of [49]) or, alternatively, H2S [50]. As zero-valent the sul¢de re£ected such environmental factors sulfur can be produced by oxidation of H2S, the rather than the variation of the N34S values of rate of transformation from monosul¢de to pyrite the source sulfate. As the deviations from the increases with increasing degree of oxidation [49]. line in Fig. 8 appear to be restricted to samples Oxidation states a¡ect the ratios of acid-insoluble collected far from the P^Tr boundary inferred by to total sul¢de in sedimentary rocks and, more- [38], the N34S values of the source sulfate might be over, the N34S values of solid sul¢de in sedimen- assumed to be relatively constant over the interval tary rocks. As hydrogen sul¢de would be re-oxi- around the P^Tr boundary. Therefore, we can dized more quickly to elemental sulfur or sulfate discuss the environmental factors, such as redox under more oxygenated conditions, some of the state and pHof pore waters, around the P^Tr sulfur species in oxygenated pore waters could boundary based on the sul¢de N34S values. escape to the bottom water before transformation The ratio of acid-insoluble sul¢de to total sul- to solid sul¢de. This loss of sulfur species could

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cause isotopic fractionation between solid sul¢de stored in sediment and source sulfate that initially 75 existed in pore water, whereas N34S values of stored sul¢de would be expected to be equal to those of source sulfate when almost all sulfate has Soil 70 been converted to solid sul¢de. In freshwater environments, N34S values of sul¢de accumulated in (wt%)

2 highly anoxic bottom waters are less di¡erent from parental sulfate than those in less anoxic SiO 65 waters [51]. The release of sulfur species might reduce the N34S values of the stored sul¢de. Con- sequently, sul¢de with a lower N34S value could be 60 produced under more oxygenated conditions (i.e. Particulate conditions corresponding to a higher Spyrite/Stotal ratio). This could have caused the negative corre- N34 18 lation between the S values and Spyrite/Stotal ratios, as seen in Fig. 8. As stated above, Spyrite/Stotal ratios can also be a¡ected by the pHof pore water in sediments, 16 besides the e¡ects of the redox state. The formation of pyrite is enhanced at low pH [52], whereas

(wt%) precipitation of monosul¢de is enhanced at high 3 pH [53]. Therefore, S /S ratios should be- O 14 pyrite total 2 come higher in the case of sedimentation in waters Al of relatively low pH. In addition, the pH of pore waters in sediments may a¡ect the isotopic com- 12 position of stored sul¢de. As hydrogen sul¢de is converted to monosul¢de more slowly in low-pH than in high-pHwater, some sulfur species in the low-pHpore water could escape to the bottom 2 water before its transformation to a solid sul¢de was completed. This loss of sulfur species could be the cause of isotopic fractionation between stored sul¢de and source sulfate, as seen in Fig. 8.

MgO (wt%)MgO 6

Fig. 9. Concentrations of SiO2 (top panel), Al2O3 (middle panel), and MgO (bottom panel) plotted against those of Fe2O3 for samples straddling the P^Tr boundary. Data after Hancox et al. [38]. Gray and open triangles represent data for the A series and corresponding to the Permian and Trias- sic sequences, respectively. Gray and open circles represent 0 2 4 6 8 data for B series samples, corresponding to Permian and Tri- assic sampling intervals, respectively. Rectangular and small Fe2O3 (wt%) cross symbols represent data of various granites [75^79] and tonalite^trondhjemite^granodiorite (TTG) gneisses [80^82] from the Kaapvaal Craton, respectively. Large diagonal and linked cross symbols represent data for a world average of river-suspended matter [59] and for soil compositions [60].

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Relatively low N34S values are observed in the cluded as the sul¢de concentrations in the P^Tr studied samples at and close to the P^Tr bound- boundary samples are similar to those in samples ary (i.e. B30.5A, B30.5B, B30.5C, B-OA, and with marine C/S ratio, although the conversion B-OC; Fig. 7c), indicating that sul¢de sedimenta- e⁄ciency of solid sul¢de from hydrogen sul¢de tion at the boundary occurred under more oxy- might have been reduced by the e¡ect of low genated conditions or lower pHthan those below pH [53] at boundary time. Therefore, the high and above the boundary. As sul¢de and organic sulfate input was most likely not limited only to matter in sediments should be oxidized more the period characterized by a marine C/S ratio, quickly under more oxygenated conditions, con- but may have extended across the P^Tr boundary. centrations of sul¢de and organic matter in the As most sulfate ions are produced through 3þ 2þ sediments should be lower under more oxygen- reactions such as FeS2+14Fe +8H2OC15Fe 23 þ C 2þ ated conditions. However, the sul¢de and organic +2SO4 +16H , FeS2+7/2O2+H2O Fe + 23 þ C þ 23 concentrations of the boundary samples are not 2SO4 +2H and SO3+H2O 2H +SO4 ,itis lower than those of the samples below and above. likely that the sulfate supplied to the freshwater Therefore, it is unlikely that the boundary sam- related to our samples was also accompanied by ples were produced under more oxygenated envi- acid. As soils have an acid-neutralizing ability, the ronments than the samples below and above the pHof pore water can be maintained as long as boundary. On the other hand, acidity of fresh- acid concentration does not exceed the acid-neu- water induced by sulfuric acid may be accompa- tralizing capacity of soil. The sul¢de accumulation nied by enhanced accumulation of sul¢de and or- under low-pHconditions at the P^Tr boundary ganic matter as presently observed in polluted means that the acid supplied to the freshwater lakes [54], indicating that low N34S values at the might have exceeded the acid-neutralizing ca- P^Tr boundary were induced by acidity rather pacity of soil. As the low N34S values, and there- than oxygenation. fore low-pHconditions, are restricted to the peri- The C/S ratios in all but one of the samples od around the P^Tr boundary, the supply of acid across the presumed P^Tr boundary (i.e. samples (and sulfate) may have reached a maximum at the B30.5A, B3OB, B3OA, and B+1.0) are higher P^Tr boundary. than marine ratios, which are unlikely to be the result of sulfate de¢ciency observed in normal 5.3. Cause of high sulfate input to freshwater freshwater environments. In acidic lake sediments, before and at the P^Tr boundary the reduction of Fe(III) is the dominant electron- accepting process [55], and the reduction of sul- Three plausible causes (i.e. enhanced weather- fate is restricted to the sedimentation zone of ele- ing, bolide impact, and enhanced volcanic activ- vated pHand relatively low ferric (hydro)oxide ity) for the introduction of high sulfate to the [56,57]. This inhibition of sulfate reduction is freshwater at and just before the P^Tr boundary thought to be a result of substrate limitation can be tested against our results. [58], because activity of Fe(III) reducers sup- presses that of sulfate reducers for electron donors 5.3.1. Increase of weathering intensity in acidic water. C/S ratios for acidic environments Hancox et al. [38] reported chemostratigraphic should be higher than marine values, because data for the same sample suites studied here. As some organic carbon is consumed by Fe(III) re- shown in Fig. 9, the elemental compositions of ducers. Therefore, for low-pHenvironments, C/S these samples can be explained by mixing of three ratios higher than marine values do not indicate components, namely river-suspended matter (‘par- low sulfate concentrations in pore water in which ticulate’), soil, and igneous materials (especially, sulfate reduction occurred, but rather that the sul- granitoids). Permian sedimentary rocks are mix- fate concentrations were higher at the time of de- tures of ancient particulates and soils, both of position of the P^Tr boundary samples than those which were produced by weathering [59,60] and, below and above the boundary. This can be con- therefore, are depleted in mobile elements such as

EPSL 6496 8-1-03 Cyaan Magenta Geel Zwart 112 T. Maruoka et al. / Earth and Planetary Science Letters 206 (2003) 101^117

K, Mg, and Ca relative to average continental preserved in sediments before developing into par- crust. Triassic sedimentary rocks are depleted in ticulate materials. Therefore, the maturity of a particulate- and soil-derived components, and are sediment can be positively correlated with degree (compared to Permian samples) enriched in less- of weathering. This implies that the concentra- weathered igneous materials, which is consistent tions of elements such as Si, Al, Fe, and Mg in with the observation that average sample grain the Permian samples re£ect a speci¢c degree of size is relatively larger in the Triassic than in the weathering. If the sulfate of freshwater was sup- Permian samples. This observation has been inter- plied only due to chemical weathering of parent preted (for sections in the southern main Karoo rock, the accumulation of biological sul¢de Basin) as the result of a change in £uvial style should also re£ect weathering intensity, and sul- from meandering to low-sinuosity channels [37]. ¢de concentrations should be correlated with Soils in freshwaters may lose mobile compo- abundances of selected major elements. However, nents and develop into suspended particulate mat- no correlation between the concentrations of Si, ter (SPM), which mainly comprises clay minerals Al, Mg and Fe and sul¢de concentrations can be and hydrous oxides enriched in elements such as recognized for any of our samples, even for the Al, Fe, and Mg relative to their parent soils. Permian samples (Fig. 10). Therefore, it is un- (Note: Mg is a highly mobile element during likely that the enhanced sulfate input was induced weathering of primary igneous rocks; however, by a change of environmental factors, such as the loss of magnesium is generally less than that temperature and CO2 concentration in the atmo- of silicon for weathering of soils [61].) If the sphere, which control weathering intensity. Thus, weathering intensity is low enough, soils can be we conclude that the sulfur observed as the en-

0.2 (a) (b) (c) (d) 0.15

0.1 S (wt %) 0.05

0 65 70 75 13 14 15 16 17 246 1 2

SiO2 (wt %) Al2O3 (wt %) Fe2O3 (wt %) MgO (wt %) 0.2 (e) (f) (g) (h) 0.15

0.1 C (wt %) 0.05

0 65 70 75 13 14 15 16 17 246 1 2

SiO2 (wt %) Al2O3 (wt %) Fe2O3 (wt %) MgO (wt %)

Fig. 10. Sulfur concentrations plotted against (a) SiO2, (b) Al2O3, (c) Fe2O3, and (d) MgO, and carbon concentrations plotted against (e) SiO2, (f) Al2O3, (g) Fe2O3, and (h) MgO for bulk samples from P^Tr boundary sections (major element data after [38]). Gray and open circles represent data for the Permian and Triassic samples of series B, respectively.

EPSL 6496 8-1-03 Cyaan Magenta Geel Zwart T. Maruoka et al. / Earth and Planetary Science Letters 206 (2003) 101^117 113 hanced sul¢de accumulation below the P^Tr that of the V2 Ma Balfour Formation (40 m boundary was directly induced by, e.g., acid [38,64]), which includes the section with the high rain, through a bolide impact or a volcanic erup- sul¢de accumulation. This does not mean that the tion, rather than by secondary environmental high sulfate supply to the freshwater was not re- changes that may have been induced by such an lated to the Siberian £ood volcanism, because it is event. not necessary that the period of high sul¢de accumulation coincides with the entire period of the 5.3.2. Bolide impact Siberian £ood eruption, as discussed below. Asteroids and comets that are vaporized during The original volume of the Siberian £ood ba- hypervelocity impact events can inject large salts has been estimated at 3.9 million km3 ([65]), masses of S into the stratosphere (from the bolide and the duration of the volcanism has been esti- and the target), to be converted to sulfate aerosol mated at less than 0.6 Ma [11]. Based on these and, eventually, sulfuric acid rain (e.g. [25,26]). As values, the average annual eruption volume can parameters such as temperature distribution and be estimated as 6.5 km3/a; such an amount of concentrations of trace gases, which are necessary basaltic magma should have contained less than 12 to calculate the residence times of SO2 in the stra- 49U10 g (49 Mt) sulfur, as estimated from S tosphere [62,63], are not well known for the Per- concentrations in FeO-rich (and S-rich) basaltic mian stratosphere, the residence time of SO2 in glasses (2500 ppm [66]; note: dissolution of sulfur the late Permian stratosphere is uncertain. More- in basaltic magmas is strongly dependent on melt over, the time period corresponding to about 5 cm FeO content (e.g. [66^68])). Even if the entire 49 of the section characterized by high sul¢de accu- Mt sulfur in the magma were released from the mulation (from 5 cm below the P^Tr boundary to magma upon the eruption, this amount is still less the boundary) is also uncertain, because we do than the recent anthropogenic £ux of sulfur (67 not have chronological information for these sam- Mt [69] or 95 Mt [70]). As the current anthropo- ples. Therefore, it is not clear whether the 5 cm genic SO2 emission apparently does not cause a section (from 5 cm below the P^Tr boundary to global e¡ect on sul¢de sedimentation, the SO2 the boundary) was laid down during a time period from volcanism with an average eruption volume 3 equivalent to the residence time of SO2 in the at- of 6.5 km /a is also unlikely to have had any mosphere after an impact event or not. However, global e¡ects. Therefore, the enhanced sulfate in- as the samples with the high sul¢de concentration put to the freshwater could have been induced do not display any petrographic and geochemical during times of enhanced eruption activity, or it features related to an impact event [38], it is un- might have been related to the decomposition of likely that a bolide impact caused the high sulfate sedimentary deposits by the strong volcanic activ- input to the freshwater. ity [71]. The Siberian Trap magma ascended through 5^5.5 km of sediments that contained 5.3.3. Increased volcanic activity abundant anhydrite [11], and the sulfur in the As sulfuric acid could be produced from SO2 in sedimentary rocks might have become entrained volcanic gases, enhanced sulfate input could be into the magma of the ¢rst stage of the Siberian induced by enhanced volcanic activity. As the volcanism [11]. Therefore, it is most likely that the Siberian £ood volcanism seems to have been co- enhanced sulfate input at and just before the P^Tr incident with the P^Tr boundary [12], this £ood boundary was related to the Siberian £ood volca- volcanism might have induced the enhanced input nism, either during maximal eruption activity or of sulfate. due to decomposition of sedimentary deposits. The duration of high sul¢de accumulation likely was much shorter than the total period of 5.4. Cause of enhanced organic accumulation Siberian Trap activity (i.e. 9 0.6 Ma [11]), judg- before and at the P^Tr boundary ing from a comparison of the thicknesses of the high-sul¢de accumulation zone (V0.05 m) and An increase of organic accumulation occurred

EPSL 6496 8-1-03 Cyaan Magenta Geel Zwart 114 T. Maruoka et al. / Earth and Planetary Science Letters 206 (2003) 101^117 simultaneously with the increase of sul¢de accu- and at the P^Tr boundary. The high accumula- mulation at and just below the P^Tr boundary tion of sul¢de was induced by enhanced input of (Fig. 7). It is unlikely, however, that organic ma- sulfate to the freshwater environment. Although terial was supplied directly by a volcanic (or im- local oxygen de¢ciency also can induce the en- pact) source. As the abundances of the organic hanced accumulation of sul¢de, it cannot explain material are not correlated with the concentra- the low C/S ratio observed for the samples with a tions of major elements that are controlled by high sul¢de concentration. This enhanced input of the weathering intensity (Fig. 10), weathering sulfate cannot be explained by changes of weath- does not seem to have been the cause for the ering-related environmental factors, such as tem- high organic accumulation at and just below the perature and CO2 concentration in the atmo- P^Tr boundary. sphere, because sul¢de concentrations do not High sul¢de accumulation can cause eutrophi- correlate with concentrations of major elements cation and, therefore, high productivity of organic that are controlled by weathering. Therefore, the material [54]. This can be induced by release of sulfate input to the freshwater was enhanced di- phosphorus from sediments to bottom water [72]. rectly by acid rain after a short-time event such as The formation of iron sul¢de should diminish the bolide impact or intense volcanic activity. So far P-binding ability of sediments, as phosphorus no convincing evidence in favor of impact has loses its binding partner (i.e. ferric complex or been found in rocks of P^Tr age. On the other ferric (hydr)oxides [73]) due to the formation of hand, the eruption of the Siberian Traps is coeval iron sul¢de. As phosphorus is an important nu- with the P^Tr boundary [12], and this volcanism trient that controls primary productivity of fresh- is believed to have involved SO2-rich eruption water, a strong release of P maintains a high level [11]. Therefore, the high sul¢de accumulation of organic productivity, which, in turn, enables demonstrated here was most probably induced intensive sulfate reduction [54]. This phenomenon by acid precipitation induced by the Siberian can explain high carbon accumulation accompa- £ood volcanism. nied by high sul¢de accumulation, as seen below Enhanced organic accumulation accompanied and at the P^Tr boundary. Although low oxygen by strong sul¢de accumulation before and at the availability, which could have occurred locally in P^Tr boundary could have been caused by eutro- the freshwater, and the die-o¡ of plants, which phication, possibly induced by the enhanced sul- might have been involved in the P^Tr mass ex- ¢de accumulation. Formation and precipitation of tinction [37], also can cause enhanced organic ac- iron sul¢de can result in a lack of binding part- cumulation, these two phenomena cannot explain ners for phosphorus, leading to release of P into the low C/S ratios observed for the samples with freshwater [72], which might have been the cause high sul¢de and organic concentration. Therefore, of eutrophication in freshwaters before and at the it is most likely that the organic accumulation was P^Tr boundary. enhanced by eutrophication induced by high sul- ¢de accumulation. Acknowledgements

6. Conclusions We would like to thank Johan Neveling and Dion Brandt for assistance during the ¢eld Sulfur and carbon concentration data, and sul- work. We thank Yukio Isozaki and Douglas Er- fur isotopic compositions, for the sedimentary win for their helpful comments and critical re- rocks straddling the inferred position of the ter- views. This work was supported by the Austrian restrial P^Tr boundary at two sites in the north- Science Foundation, FWF project Y58-GEO (to ern Karoo Basin of South Africa show that the C.K.). The research of P.H. and W.U.R. is sup- accumulation of sul¢de produced by sulfate-reported by the National Research Foundation of ducing bacteria was relatively enhanced just below South Africa and through a grant from the Uni-

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