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IGCP591 special issue - The Homerian (late Wenlock,

Silurian) carbon isotope excursion from Perunica: does dolomite control the magnitude of the carbon isotope

excursion?

Journal: Canadian Journal of Earth Sciences

Manuscript ID cjes-2015-0188.R1

Manuscript Type: Article

Date Submitted by the Author: 03-Jan-2016

Complete List of Authors: Fryda, Jiri; Czech University of Life Sciences, Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kamýcká 129, Praha 6 – Suchdol, 165 21 Frydova , Barbora ; Czech University of Life Sciences Prague, 1Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kamýcká 129, Praha 6 – Suchdol, 165 21

Keyword: Silurian, Homerian carbon isotope excursion, Perunica, dolomite, Chemostratigraphy

https://mc06.manuscriptcentral.com/cjes-pubs

Canadian Journal of Earth Sciences

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The Homerian (late Wenlock, Silurian) carbon isotope excursion from Perunica: does 1

dolomite control the magnitude of the carbon isotope excursion? 2

3

Jiří Frýda1,2 and Barbora Frýdová1 4

5

1Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kamýcká 6

129, Praha 6 – Suchdol, 165 21, Czech Republic, and 2Czech Geological Survey, Klárov 7

3/131, 118 21 Prague 1, Czech Republic 8

Corresponding author (e-mail: [email protected]). 9

10

11

Abstract: The δ13Ccarb records from two geographically close sections of the shallow-water 12

Kozel Limestone Member (late Wenlock Motol Formation; Perunica microplate) significantly 13

differ in the magnitude of the Homerian carbon isotope excursion as well as in their dolomite 14

content. The present paper tests a hypothesis as to whether a difference of about 2‰ in the 15

magnitude of the δ13Ccarb anomaly is caused by the different content of dolomite, which could 16

be enriched in both 13C and 18O relative to coexisting calcite, as has been suggested by 17

experimental data. The new data obtained reveal that the δ18O composition of calcite and 18

dolomite was probably controlled by the pore fluid composition during limestone diagenesis 19

and that both carbonates seem to be close to equilibrium in oxygen isotope composition. On 20

the other hand, the δ13C values of dolomite are similar to those of calcite and thus the carbon 21

isotope composition of both carbonates was probably determined by the precursor carbonate 22

composition. Moreover, the values of δ13Cdolomite/ δ13Ccalcite ratios as well as their variability 23

suggest that both calcite and dolomite did not reach equilibrium in their carbon isotope 24

composition. Whole-rock, mineralogical and C and O isotope data clearly show that dolomite 25

is not the cause for the differences in magnitudes of the δ13C records observed between 26

dolomite-bearing and dolomite-lacking shallow-water limestone successions. The question 27

as to whether the observed differences in the δ13C records of the studied sections across the 28

Homerian carbon isotope excursion were controlled by the dependence of sea water 29

composition on water depth and/or proximity to shoreline, or if the δ13C values were later 30

affected by secondary processes during limestone diagenesis is still unsolved. 31

Résumé: xx 32

Key words: Silurian, Homerian, carbon isotope excursion, Perunica, dolomite 33

34

35

Introduction 36

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Our knowledge of the carbon isotope perturbations in Silurian marine ecosystems has 37

considerably increased during the last two decades (see summaries in Loydell 2007, 38

Munnecke et al. 2010, and Cramer et al. 2011). Evidence for their global distribution is now 39

indisputable as well as for their occurrence in different marine environments. The Silurian 40

carbon isotope anomalies have been documented in both inorganic and organic carbon 41

marine reservoirs. Nowadays all major Silurian carbon isotope excursions are recorded from 42

shallow-water platform environments to deeper pelagic depositional settings including 43

graptolite shales (see review in Cramer et al. 2011). Increasing data support earlier 44

observations (Kaljo et al. 1997, Noble et al. 2005, Melchin and Holmden 2006, and Loydell 45

2007) on the shoreward increase of the magnitude of carbon isotope anomalies. Numerous 46

conceptual models have been offered to explain major Paleozoic carbon isotope excursions 47

over the past two decades, stemming from detailed sedimentological investigations of the 48

potential role played by climate, sea level, and seawater circulation as factors influencing 49

stratigraphic variation in sedimentary δ13C values. Recently Kozłowski and Sobien (2012) 50

and Kozłowski (2015) published a new model which links the Ludfordian (late Silurian) 51

carbon isotope excursion, appearing to be the highest magnitude positive δ13Ccarb excursion 52

in the post-Cambrian Phanerozoic, with coeval regressions and development of evaporitic 53

areas in a dry climate, which would contribute elevated dolomite amounts to the basin. 54

Occurrence of dolomite is rather common in the Silurian limestone successions recording 55

carbon isotope excursions. 56

This is true also for the Homerian (late Wenlock, Silurian) carbon isotope positive anomaly 57

which is characterized by a double-peaked excursion and distinct faunal overturn (see 58

summaries in Loydell 1998, 2007, Cramer et al. 2011, 2012 and Jarochowska et al. 2015). 59

Two bioevents occurred just before or coincident with the onset of the carbon isotope 60

excursion: the first described affected graptolites and is known as the “Große Krise” or 61

lundgreni Event (Jaeger 1959, 1991, Koren’ 1991; Štorch 1995; Kaljo et al. 1996; Lenz & 62

Kozłowska-Dawidziuk, 2001, 2002; Porębska et al. 2004; Lenz et al. 2006; Noble et al. 2005, 63

2012) and the second to be described affected conodont faunas and has been named the 64

Mulde Event (Jeppsson et al. 1995, Calner & Jeppsson 2003, Jeppsson & Calner 2003, 65

Calner et al. 2006, but see also Jarochowska and Munnecke, 2015). 66

The Homerian carbon isotope excursion has hitherto been recorded from Baltica (Samtleben 67

et al. 2000; Calner & Jeppsson 2003; Calner et al. 2006; Kaljo et al. 2007; Calner et al. 2012; 68

Jarochowska et al. 2015), Avalonia (Corfield et al. 1992; Marshall et al. 2012), Laurentia 69

(Cramer et al. 2006) and peri-Gondwana (Frýda and Frýdová 2014). The first peak of the 70

double-peaked excursion typically reaches a δ13Ccarb value of about +3.5‰ as summarized 71

by Cramer et al. (2011) and Saltzman and Thomas (2012), and the second peak is always 72

slightly lower. Nevertheless there is increasing evidence for a much higher magnitude first 73

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peak from limestone successions deposited in shallow-water marine environments often 74

containing dolomite (e.g., Kaljo et al. 2007, Marshall et al. 2012). 75

Here we report a new δ13Ccarb record from shallow-water limestones of the Prague Basin 76

across the first peak of the Homerian carbon isotope excursion reaching up to +4.5‰. The 77

new isotope data come from the Kozel Syncline area which is located only about 1.5 km from 78

the previously described Kozel section (no. 760) recording a much lower δ13Ccarb anomaly 79

(Frýda and Frýdová 2014). Unpublished sedimentological data suggest that the limestones 80

with higher δ13Ccarb values (Kozel Syncline section no. 244JF) were deposited in slightly 81

shallower depth than those having lower δ13Ccarb values (Kozel section no. 760). Both 82

sections record the same carbon isotope anomaly but its magnitude differs by about 2‰ δ13C 83

(Fig. 1). The only distinct difference between the two sections is the high dolomite content in 84

the section with the higher δ13C values. The effect of dolomite on carbon isotope anomalies 85

in marine ecosystems is not yet well evaluated (see discussion in Kozłowski 2015). In 86

addition, numerous theoretical and experimental studies have revealed that dolomite in 87

equilibrium with calcite is enriched in both 13C and 18O relative to coexisting calcite (see 88

summary in Horita 2014). The main aim of the present paper is to test a hypothesis: whether 89

the distinct difference in the magnitude of the first peak of the Homerian carbon isotope 90

excursion observed in these two shallow-water limestone successions from the Prague Basin 91

is caused by a difference in dolomite content. 92

93

Geological settings 94

The Homerian (late Wenlock) strata of the Barrandian form the upper part of the Motol 95

Formation [for definition and stratigraphical range of the Motol Formation see Kříž (1975)]. 96

According to its present definition, the Motol Formation (thickness 80 m to 300 m) comprises 97

the uppermost Telychian (Llandovery), the entirety of the Wenlock, and possibly the 98

lowermost Gorstian (Kříž 1992, 1998, Frýda and Frýdová 2014). The lower part of the Motol 99

Formation is sedimentologically uniform and is represented mostly by calcareous clayey 100

shales. Facies distribution in the upper part of the Motol Formation was strongly influenced 101

by synsedimentary tectonic movements and volcanic activity (Bouček 1934, 1953; Horný 102

1955a,b, 1960; Kříž 1991). 103

During the Homerian the high activity of several volcanic centres (i.e., Svatý Jan Volcanic 104

Centre, Řeporyje Volcanic Centre and Nová Ves Volcanic Centre) created shallow areas 105

generating a complex facies pattern from very shallow intertidal limestones to deeper water 106

shales deposited in an anoxic environment (see for details Bouček 1953; Horný 1955a,b,c, 107

1960, 1962; Fiala 1970; Kříž 1991, 1992, 1998; Štorch 1998). The central area of the Svatý 108

Jan Volcanic Centre was emergent during the latest Wenlock and Ludlow forming a volcanic 109

island. Shallow-water environments surrounding the island gave rise to mostly crinoidal–110

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coral–brachiopod–bryozoan grainstones, rudstones and packstones containing a variable 111

amount of tuffitic and/or dolomitic components. The latter shallow-water limestone unit was 112

formally established as the Kozel Limestone Member of the Motol Formation (Frýda and 113

Frýdová 2014). 114

The present study focuses on two sections within the Kozel Limestone Member which are 115

about 1.3 km apart. Both sections are situated on the left (north) bank of the Berounka River. 116

The first section (49.9561064N, 14.0977517E) known as “Kozel” was named by Kříž et al. 117

(1993) as the Kozel section (no. 760) and it has been intensively studied since the beginning 118

of the last century (see Kříž 1991, 1992, 1998; Kříž et al. 1993 for references). The most 119

detailed description of the section was given by Kříž (1992), Kříž et al. (1993) and Dufka 120

(1995), who discussed its lithology, faunal communities and biostratigraphy. The second 121

section (49.9536697N, 14.1149714E) named here as section no. 244JF occurs in the so-122

called "Kozel Syncline" situated about 1.3 km E of section no. 760, and 1 km W of the 123

estuary of the Kačák brook (Fig. 1). The Kozel Syncline area was mentioned in the past as 124

an important fossil locality but has never been studied in detail. 125

126

Methods 127

Limestone samples (about 5 kg each) from the Kozel Syncline section (no. 244JF) were 128

sampled in order to investigate the chemostratigraphical record across the first peak of the 129

Homerian (late Wenlock, Silurian) carbon isotope excursion. This new sampling campaign 130

included primarly 56 limestone samples for carbon and oxygen isotope analyses, 36 of which 131

were selected for thin-section study and whole-rock analyses. Grainstones, rudstones, and 132

packstones represent the dominant limestone lithologies of the lower part of the Kozel 133

Limestone Member in the Kozel Syncline section. 134

A few milligrams of rock powder from each sample were recovered with a dental drill from cut 135

and polished slabs. The mineralogical composition of the rock powder was controlled by X-136

ray powder diffraction analysis using a Philips X´Pert diffractometer. Only calcite samples 137

having no dolomite or traces of dolomite were used for carbon and oxygen isotope analyses. 138

In three samples calcite and dolomite grains were mechanically separated under the 139

microscope. About 1 kg of each of the 36 whole-rock samples was, after cleaning in the 140

ultrasonic bath, dissolved in cold and very weak HCl (1:20) to separate fine grains of 141

dolomite from the limestone. The purity of the dolomite was tested by X-ray powder 142

diffraction analysis and only calcite-free dolomite samples were used later for carbon and 143

oxygen isotope analyses. 144

The carbonate powder (calcite or dolomite) was reacted with 103% phosphoric acid at 25°C 145

for about 24 hours (calcite powders) or 48 hours (dolomite powders). The carbon and oxygen 146

isotopic composition of the released CO2 was meassured with a Thermo Delta 5 mass 147

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spectrometer in dual inlet configuration. The meassured δ18O value of dolomite was 148

corrected according to the equations published by Becker and Clayton (1976) and 149

Rosenbaum and Sheppard (1986). All values are reported in ‰ relative to the V-PDB by 150

assigning a δ13C value of +1.95 ‰ and a δ18O value of +2.20 ‰ to NBS 19. Accuracy and 151

precision were controlled by replicate measurements of laboratory standards and were better 152

than ±0.1‰ for both carbon and oxygen isotopes. 153

Whole-rock samples (about 2 kg) were, after cleaning in the ultrasonic bath, powdered using 154

an agate mill and then homogenized. Two small samples of about 5g from each powdered 155

whole-rock sample were selected for further chemical analyses. The first sample was 156

leached by weak HCl (1:1) at room temperature to dissolve preferably carbonates, and the 157

second sample was dissolved using a mixture of H2SO4, HNO3 and HF at a temperature of 158

200°C in a closed Teflon box. Both solutions were later analysed for their Ca, Mg, Fe, and 159

Mn contents by the atom absorption spectrometry method using a Perkin-Elmer AAnalyst 160

100 spectrometer. Analyses of the chemical composition of calcite and dolomite were 161

performed by CAMECA SX100 microprobe under 15 kV and 4 nA using spectrometers with 162

LIF, TAP and PET crystals and certified carbonate standards for Ca, Mg, Fe, Sr and Mn. 163

The non-parametric rank-order Spearman´s as well as Kendall´s tests (including calculation 164

of the Spearman´s and Kendall´s correlation coefficients and t-test of their significance) were 165

used for testing the correlation of meassured geochemical data. Because of the latter data 166

are not normally distributed, the non-parametric Kruskal-Wallis test was used for testing that 167

samples were taken from population with the same mean. 168

169

170

Results 171

δ13C chemostratigraphy 172

The δ13Ccarb data from limestone samples from the Kozel section (no. 760) were reported by 173

Frýda and Frýdová (2014). The new δ13Ccarb data from the Kozel Syncline section (no. 174

244JF) are summarized in Figures 1 and 2. The first three samples from loose limestone 175

blocks enclosed by tuffitic beds below the base of the Kozel Limestone Member range from 176

1.3‰ to 1.6‰. By contrast all δ13Ccarb values from the overlying limestone beds belonging to 177

the lower part of the Kozel Limestone Member are much higher. The δ13Ccarb values rise 178

rapidly from +2.7‰ in the first bed of the Kozel Limestone Member to +4.5‰ about 5 m 179

above the base of the Kozel Limestone Member. The δ13Ccarb values of fourteen samples 180

from the about 2.4 m thick interval just above the highest δ13C value of the the Kozel 181

Syncline section (no. 244JF) have a decreasing trend ranging from +4.5‰ to +3.3‰ (Fig. 1). 182

In the subsequent interval, about 0.3 m thick, the δ13C values again increase to +4.1‰. The 183

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δ13Ccarb values in the overlying limestone beds with a total thickness of about 6 m fall to a 184

value of +2.5‰ (Figs 1 and 2). 185

186

Whole rock chemistry 187

Whole rock analyses of the limestone samples from the Kozel Syncline section (no. 244JF) 188

revealed a rather high variability in Mg content. Increased Mg content was recorded from the 189

interval about 5.5 to 8 m from the base of the Kozel Limestone Member and in the two 190

stratigraphically highest samples (Fig. 2A) in which the Mg content reached values of about 8 191

and 5.5 wt.% respectively. Chemical analyses of solutions formed after dissolving of whole 192

rock powder in 1:1 HCl gave slightly lower Mg contents (Fig. 2A). X-ray analyses of residua 193

after dissolution revealed that dolomite was also dissolved. The Mg/Ca molar ratio of most of 194

whole rock samples varies from about 0.01 to 0.1 (Fig. 2B). Only one Mg/Ca ratio value is 195

below this range (0.005) and it belongs to one of limestone clasts enclosed by the tuffites 196

below the base of the Kozel Limestone Member (Fig. 1). The highest Mg/Ca ratio values, 197

0.333 and 0.237 respectively, were found in the two stratigraphically highest samples. 198

199

Calcite and dolomite 200

The new carbon and oxygen isotope data from calcite and dolomite from the Kozel Syncline 201

(section no. 244JF) are summarized in Figure 3. There is a statistically significant difference 202

between the δ13C values of the calcite samples (Kruskal-Wallis test; p<0.001) from the two 203

studied sections of the Kozel Limestone Member (i.e., sections no. 244JF and no. 760). 204

However, there is no statistically significant difference between the δ13Ccalcite and δ13Cdolomite 205

values of samples coming from the Kozel Syncline section (Kruskal-Wallis test; p>0.05) (Fig. 206

3A). On the other hand, all three δ18O datasets [i.e., δ18Ocalcite values from both sections (no. 207

760 and no. 244JF) as well as δ18Odolomite values from section no. 244JF] are significantly 208

different (Fig. 3B). Detailed comparison of carbon isotope fractionation between coexisting 209

calcite and dolomite grains from three whole-rock samples (including three replicates), which 210

were mechanically separated under the microscope, revealed that the δ13C values of 211

dolomite may be slightly lighter or heavier than the δ13C values of coexisting calcite (Fig. 3C). 212

In addition, available data suggest that the carbon isotope fractionation between calcite and 213

dolomite depends on the Mg content of the whole rock and thus on the content of dolomite 214

(Fig. 3D). However, because of the low number of studied samples (n=9) the latter 215

relationship is not quite robust. 216

Microprobe analyses showed that calcite contains on average about 1 molar percent of 217

MgCO3, thus having Mg/Ca ratio of about 0.01. Analyses also showed that dolomite is Ca 218

enriched and thus far from ideal stoichiometry CaMg(CO3)2. Average dolomite composition 219

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may be expressed by the formula Ca1.10Mg0.82Fe0.08(CO3)2, which corresponds to a Mg/Ca 220

molar ratio of about 0.75 (Fig. 2B). 221

222

Discussion 223

The main aim of the present paper is to test the hypothesis that a high dolomite content may 224

considerably increase the δ13C values of limestone samples coming from the dolomite-225

bearing Kozel Sycline section (no. 244JF). X-ray diffraction data, whole-rock analyses (Fig. 226

2A) and thin section studies revealed that dolomite is the main Mg-bearing phase and 227

therefore the Mg content of whole-rock samples may be used for an estimation of dolomite 228

content. If average compositions of calcite and dolomite are used (Fig. 2B), then the dolomite 229

content in each whole-rock sample may be calculated. The latter estimation revealed that the 230

dolomite content in the majority of studied whole-rock samples is less than 15 molar % with 231

average and median values of 5% and 4% respectively. The highest dolomite content was 232

found in the two stratigraphically highest samples which contain about 44% and 30% 233

dolomite respectively. The relationships of the δ13C values and the Mg content in the whole-234

rock samples (Fig. 2D) as well as of the δ13C values and Mg/Ca molar ratios (i.e., estimated 235

dolomite content) (Fig. 2C) were analysed to test whether dolomite is responsible for the high 236

δ13C values. The analyses showed that there are no statistically significant correlations 237

amongst the above-mentioned variables (for both non-parametric rank-order Spearman´s as 238

well as Kendall´s tests is p>>0.05; Figs 2B and 2D). In addition, the highest δ13C values 239

come from samples with very low Mg and thus dolomite contents (Fig. 2). These results 240

suggest that the high dolomite content in samples from the Kozel Syncline section (no. 241

244JF) does not increase the δ13C record across the first peak of the Homerian carbon 242

isotope excursion (Fig. 1). 243

Study of δ13Ccalcite and δ13Cdolomite compositions resulted in a similar conclusion. There is no 244

statistically significant difference between the δ13Ccalcite and δ13Cdolomite values of samples from 245

the Kozel Syncline section (no. 244JF) (Fig. 3A). As shown in Figure 3C the dolomite has 246

similar δ13C values to coexisting calcite but its δ13C values may be slightly lighter or heavier 247

than the δ13C values of coexisting calcite. The first peak of the double-peaked Homerian 248

excursion reaches a δ13Ccarb value of about +3.3‰ in the Kozel section (no. 760), similar to 249

the values noted by Cramer et al. (2011). The magnitude of the first peak of the Homerian 250

carbon isotope excursion at the Kozel Syncline section (no. 244JF) is, however, about 2‰ 251

higher than typically recorded values. The small difference in the δ13C values of calcite and 252

dolomite from the Kozel Sycline section cannot be responsible for this increase of about 2‰ 253

found in these dolomite-bearing limestones. Determined dolomite/calcite carbon isotope 254

fractionation values (Fig. 3D) suggest that even if any whole-rock sample was formed of pure 255

dolomite, then the maximum increase of its δ13C value should be less than 0.7‰ relative to 256

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dolomite-free limestone, and thus not the observed about 2‰ difference (see Fig. 1). 257

Moreover, the samples with the highest δ13C values have a very low dolomite content 258

(compare Figs 2A and C). Taken together, the whole-rock chemistry and carbon isotope data 259

clearly reject the hypothesis that a high dolomite content is responsible for the 2‰ difference 260

in the magnitude of the first peak of Homerian carbon isotope excursion found in the two 261

studied sections. 262

The δ13C and δ18O composition of calcite and dolomite may also help to understand the 263

origin of dolomite, but one has to keep in mind that both calcite and dolomite isotope 264

signatures could be influenced by diagenetic processes during limestone burial. Diagenetic 265

carbonates formed during burial tend to show decreasing δ18O values with burial as a result 266

of many different factors, including increased temperature and evolution of the δ18O values of 267

the pore fluids. In contrast, the δ13C values of the diagenetic carbonate should not be 268

generally significantly different from the δ13C values of the original marine carbonate 269

sediment if its organic matter content is low (see summary in Swart 2015). However, both the 270

δ13C and δ18O values of carbonates are controlled by several factors. The δ13C and δ18O 271

values of marine carbonate minerals depend upon the temperature of their formation, the 272

δ13C and δ18O values of the sea water, the mineralogy of precipitating carbonate, the pH of 273

the solution, as well as on kinetic effects (e.g., Urey, 1947, Epstein et al., 1953, Emrich et al., 274

1970 and Deines et al., 1974, Zeebe and Wolf-Gladrow, 2001, and McConnaughey, 2003,). 275

In this context, the δ18O values of calcite from the two studied sections (no. 780 and no. 276

244JF) fall within the range for Palaeozoic limestones having a well-preserved primary δ13C 277

signature. For example, the δ18O values of about 40 limestone samples from the flat-topped 278

peak of the mid-Ludfordian carbon isotope excursion (i.e., from the δ13C chemostratographic 279

S-zone) having constantly rather high δ13C values of about +8‰ (see Frýda and Manda 280

2013), range from -6.5 to -4‰. The latter δ18O values are thus rather variable probably 281

because the limestone was affected locally by a different intensity of reaction with diagenetic 282

fluids. On the other hand, their constant δ13C values suggest that the carbon isotope 283

composition was not considerably affected by diagenesis. The δ18O values of limestone 284

samples coming from the Kozel section (no. 760) and the the Kozel Syncline section (no. 285

244JF) are significantly different (Fig. 3B). Samples from the Kozel (section no. 670) have 286

lighter δ18O values than those from the Kozel Sycline (section no. 244JF) which may suggest 287

a more intensive influence of secondary (diagenetic) processes on the buried marine 288

carbonates. The latter processes could theoretically also lower their δ13C values (e.g., by 289

reaction with pore fluids contaning light carbon isotope values from oxidation of organic 290

matter). 291

The δ13C and δ18O values of calcite and dolomite samples from the the Kozel Syncline 292

section (no. 244JF) revealed that there is no statistically significant difference between the 293

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δ13Ccalcite and δ13Cdolomite values (Fig. 3A), but, their δ18O values are significantly different (Fig. 294

3B). Published experimental as well as natural data showed that dolomite should be enriched 295

in both 13C and 18O in respect to coexisting calcite (Horita 2014). The negative values of δ18O 296

of the studied dolomite and calcite (Fig. 3B) suggest that both carbonates were modified 297

during diagenetic processes (e.g., Hudson, 1977). The dolomite from the Kozel Syncline 298

section is enriched in 18O relative to calcite by about 3 to 4‰ (Fig. 3), which is in good 299

agreement with experimental and natural data (see Horita 2014 for references). The δ18O 300

composition of calcite and dolomite was probably controlled by the pore fluid composition 301

and both carbonates seem to be close to equilibrium in δ18O. On the other hand, the δ13C 302

values of dolomite are similar to those of calcite and thus the carbon isotope composition of 303

both carbonates was probably determined by the precursor carbonate composition, because 304

pore diagenetic fluids generally have low carbon content. Therefore the δ13C values of 305

limestones could be considered as a good proxy for the isotopic compositions of carbonate 306

precipitated from marine water (Fig. 3). Nevertheless, the observed dependence of carbon 307

isotope fractionation between calcite and dolomite on the Mg whole-rock content and thus on 308

the content of dolomite (Fig. 3D) may suggest a weak influence of the dolomitization process 309

on the δ13C values of limestones. 310

311

Conclusions 312

The main aim of the present paper was to test the hypothesis whether the distinct difference 313

in magnitude of the first peak of the Homerian carbon isotope excursion observed in two 314

shallow-water successions of the Kozel Limestone Member was caused by differences in 315

dolomite content. Whole-rock elemental composition and isotope data clearly rejected this 316

hypothesis. On the other hand, the question as to whether the observed differences in the 317

δ13C records of the two studied sections across the Homerian carbon isotope excursion were 318

controlled only by dependence of sea water composition on water depth and/or proximity to 319

land, or whether the δ13C values were later partly overprinted by some secondary process 320

during limestone diagenesis is still unsolved. 321

322

Acknowledgements 323

This paper is a contribution to IGCP 591 and IGCP 596. The research was supported by a 324

grant from the Grant Agency of the Czech Republic (GAČR 15-133105), and a grant from the 325

Czech Geological Survey (338800). We are very grateful to D. K. Loydell who improved the 326

English and provided valuable comments on the manuscript. This paper also benefited from 327

the constructive reviews of two anonymous referees. 328

329

References 330

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Explanation of figures: 497

Figure 1. The δ13C records across the Homerian (late Wenlock, Silurian) carbon isotope 498

positive anomaly from the Barrandian area (Perunica). A - Distribution of Silurian rocks in the 499

Barrandian area, including the locations of both studied sections, the Kozel section (no. 760) 500

and the Kozel Syncline section (no. 244JF). B - The δ13C record from the Kozel section 501

(modified from Frýda and Frýdová, 2014). C - The δ13C record from the Kozel Syncline 502

section. 503

504

Figure 2. Whole-rock elemental composition and isotope data from the Kozel Syncline 505

section. A - Stratigraphical distribution of Mg whole-rock content. B - Relationship of the δ13C 506

values and Mg/Ca molar ratios. Grey bands represent composition of calcite and dolomite. C 507

- Stratigraphical distribution of δ13C values. Grey curve represents locfit regression. D - 508

Relationship of the δ13C values and the Mg content in the whole-rock samples. See text for 509

discussion. 510

511

Figure 3. A, B - The δ13C and δ18O composition of calcite and dolomite from the two studied 512

sections, the Kozel section (no. 760) and the Kozel Syncline section (no. 244JF). C - The 513

δ13C composition of coexisting calcite and dolomite from three whole-rock samples of the 514

Kozel Syncline section. Grains separated mechanically are connected by a line. D - 515

Relationship of the carbon isotope fractionation between coexisting calcite and dolomite and 516

the Mg content in the whole-rock samples. See text for discussion. 517

518

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