tents of organic carbon (more than 50%; Dean et al., 1986) ... Jenkyns, 1976; Arthur and Schlanger, 1979; ..... Arthur, M. A., Dean, W. E., and Pratt, L. M., 1988,.
Sulfur isotope records around Livello Bonarelli (northern Apennines, Italy) black shale at the Cenomanian-Turonian boundary N. Ohkouchi K. Kawamura Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan
Y. Kajiwara Institute of Geoscience, University of Tsukuba, Tsukuba 305, Japan
E. Wada Center for Ecological Research, Kyoto University, Otsu 520-01, Japan
M. Okada Department of Environmental Sciences, Ibaraki University, Mito 310, Japan
T. Kanamatsu Japan Marine Science and Technology Center, Yokosuka 237, Japan
A. Taira Ocean Research Institute, University of Tokyo, Nakano-ku Tokyo 164, Japan
ABSTRACT Sulfur isotope ratios for carbonate-hosted sulfate show a rapid increase (+6‰ to +9‰/m.y.) across the Livello Bonarelli black shale (Marche-Umbrian Apennines of Italy), which was deposited at the Cenomanian-Turonian boundary in the middle Cretaceous (93.5 Ma). The increase suggests that rates of pyrite burial increased substantially over the event, resulting in a decrease of 9%–27% in the oceanic reservoir of SO42–. The enhanced pyrite burial may have significantly increased the flux of oxygen to the atmosphere (9–27 × 1018 mol/m.y.). Furthermore, removal of the large amount of iron associated with this process might have had a profound effect on iron cycle.
INTRODUCTION Among several Cretaceous black shales, one from the Cenomanian-Turonian (C-T) stage boundary at 93.5 Ma (Gradstein et al., 1994) is locally characterized by extraordinarily high contents of organic carbon (more than 50%; Dean et al., 1986), and it is often said to represent oceanic anoxic event 2 (OAE-2) (Schlanger and Jenkyns, 1976; Arthur and Schlanger, 1979; Jenkyns, 1980; Hayes et al., 1989). Evidence of OAE-2 is widespread throughout the deposits of the Atlantic Ocean, the former Tethys ocean, and some parts of the Pacific Ocean (Thiede et al., 1982; Arthur et al., 1988). Deposition of the black shales has been attributed to several factors, including efficient preservation of organic matter due to the anoxic deep-ocean environment (e.g., Schlanger and Jenkyns, 1976), high oceanic productivity (Pedersen and Calvert, 1990), massive turnover of nutrient-rich bottom water (Vogt, 1989), and large-scale submarine magmatism (Bralower et al., 1997). However, the factor or factors primarily controlling the deposition of Cretaceous black shales are still a matter of debate. In this study we report isotopic compositions of sulfate sulfur in the rocks deposited around the OAE-2 shale that crops out at Gorgo Cerbara in the northern Apennines, Italy, and argue that the sulfur cycle of the C-T ocean should have varied with the redox conditions of the ocean. Geology; June 1999; v. 27; no. 6; p. 535–538; 3 figures.
METHODOLOGY Pulverized samples were treated with NaOCl solution and benzene to remove organically bonded sulfur and elemental sulfur, respectively. The bleached residue was then treated with dilute HCl to decompose carbonate. Hydrogen sulfide was evolved from certain acid-soluble sulfides was immediately eliminated by the N2 stream, being trapped and collected in a zinc acetate solution. Acid-soluble sulfate hosted by carbonate was dissolved into the solution and collected as BaSO4 (Kajiwara et al., 1997). The BaSO4 was converted to Ag2S by the Kiba method (Sasaki et al., 1979) and then was converted to SO2 by means of the vacuum-distillation technique (Robinson and Kusakabe, 1975). Isotopic measurements were performed by use of a Finnigan MAT Delta-E. The analytical precision for the extraction procedure, conversion to Ag2S, purification, and mass spectrometry is ±0.2‰. Previous studies of infrared spectra and X-ray absorption near-edge-structures analyses of carbonate minerals and sedimentary carbonates indicated that sulfur in carbonate rocks is not in the form of gypsum or anhydrite inclusions, but rather sulfate substituting for carbonate (Takano et al., 1980; Burdett et al., 1989; Pingitore et al., 1995). Thus, the isotopic composition of the carbonate-hosted sulfate presumably represents that of dissolved sulfate in seawater at the time of car-
bonate formation (Burdett et al., 1989; Kajiwara et al., 1997), although the detailed nature of sulfate incorporation in carbonate is not well known. To estimate the mean sedimentation rate, we determined aluminum contents as a proxy for lithogenic matter in our samples. If we assume that the mass accumulation rate of the aluminum was constant, the mean sedimentation rate of the black shale is one-fourth of that of adjacent chert and limestone sequences. As the Turonian sequence at Gubbio was estimated to have accumulated at 5.3 m/m.y. on the basis of planktonic foraminifera–based biostratigraphy (Premoli Silva et al., 1977), we estimated that the C-T event lasted ~0.8 m.y. and had a mean sedimentation rate of 1.3 m/m.y. RESULTS AND DISCUSSION The dominant lithofacies of a section at Gorgo Cerbara in the northern Apennines consist of nannofossil-rich limestone and light gray chert (Fig. 1). The distinctive Livello Bonarelli black shale (resulting from OAE-2) makes up the middle of the sequence and has a thickness of ~1.1 m. The Bonarelli black shale is found over a wide area of northern Italy and probably represents a range of bathymetric settings from the plateau to basins (Arthur and Premoli Silva, 1982). Sporadic pyrite lenses and nodules are seen within the Bonarelli horizon. A mean total 535
organic carbon (TOC) content for the Bonarelli samples is 9.8%, whereas TOC values for limestone and chert samples are 0.13%. Thus, the TOC content in the black shale is about 75 times higher relative to those of the adjacent rocks (Ohkouchi et al., 1997). The sulfur isotopic ratios of sulfate (δ34SSO4 ) in the organic-poor carbonate and chert sequence are 16.5‰ ± 2.1‰ in the Cenomanian and 20.7‰ ± 2.2‰ in the Turonian (Fig. 2). A remarkable δ34SSO4 increase of 5‰ to 7‰ was observed across the Bonarelli horizon. The positive shift of δ34SSO4 suggests that either sulfur with heavier isotopic ratios was added to the ocean or sulfur with lighter isotopic ratios was selectively removed from the seawater during the black-shale deposition. Because the C-T event lasted ~0.8 m.y., the rate of the δ34SSO4 shift at the C-T boundary is calculated to have been +6‰ to +9‰/m.y., which is the fastest rate so far reported in Earth’s history (Claypool et al., 1980; Schidlowski et al., 1983). On the geologic time scale, oceanic sulfur content represents a balance between riverine input and outputs including hydrothermal processes, evaporite formation, and microbial sulfate reduction leading to pyrite burial. Enhanced pyrite burial is the most likely cause for the positive shift of δ34SSO4 at the C-T boundary, because the ocean was estimated to have been anoxic at this time and because microbial reduction is the only process that produces large fractionation in sulfur isotopes (Goldhaber and Kaplan, 1974). We constructed a simple one-box kinetic model for isotope and mass balances of oceanic sulfur to estimate the impact of this event on the sulfur cycle (Fig. 3). In the following argument, we assume an oceanic residence time
Figure 1. Stratigraphic section that includes Cenomanian-Turonian boundary (93.5 Ma) crops out at Gorgo Cerbara in northern Apennines, Italy. Horizon of Livello Bonarelli black shale is shown as black layers in middle of this sequence. 536
Figure 2. Analytical results of δ34S of sulfate in rock samples from Gorgo Cerbara. Gray band indicates horizon of Livello Bonarelli black shale. Isotope ratios are expressed in the δ notation: δ34S = (Rsample /Rstandard – 1) × 1000 (in ‰), where R denotes 34S/ 32S ratios of sample and standard (Canyon Diablo troilite).
of sulfur of 15 m.y., the same value as that just preceding anthropogenic perturbation (Holland, 1973) before the black-shale deposition. We also assume that the removal rates of sulfur through pyrite burial and hydrothermal activity are controlled by first-order kinetics. Although the isotopic fractionation of sulfur at the time of bacterial reduction is involved in several biological and physicochemical factors, laboratory culture experiments of bacterial cells showed that fractionation generally ranges from –20‰ to –46‰ and that the faster the reduction, the smaller the fractionation (Goldhaber and Kaplan, 1974). In the Black Sea, a modern anoxic body of seawater, fractionation was reported to be –60‰ (Sweeny and Kaplan, 1980). If the isotopic fractionation was –60‰, the mean pyrite burial rate during the C-T event is calculated to have been nine times more than that of the adjacent periods, and the oceanic reservoir of sulfur was reduced by 9% at the last stage of OAE-2 (Fig. 3). If the isotopic fractionation was –20‰, the mean sulfate reduction rate was increased 28 times more, and the sulfur reservoir was reduced by 27%. If the oceanic sulfur reservoir during the Cenomanian was the same as that of present (1.3 × 106 Gt), the integrated amount of sulfur removed by the pyrite burial during OAE-2 was 0.12–0.36 × 106 Gt. One of the most impressive phenomena associated with a large amount of pyrite burial is the elevation of oxygen flux from the ocean to the atmosphere. Using the OAE-2–determined pyrite burial rate in a model described in Holland (1973), we calculate the excess O2 flux to the atmosphere during the OAE-2 as 9–27 × 1018 mol/m.y., which corresponds to 200%–600% of the average O2 flux in the Phanerozoic (Kump and Garrels, 1986). Coupling with an effect of elevated burial rate of organic carbon (Arthur et al., 1988), atmospheric O2 concentration could have significantly increased during this event. Some feedback mechanisms that compensate for this additional flux of O2 may be considered. A further implication from our sulfur isotopic results is the potential effect that enhanced sulfide burial at the C-T boundary had on the oceanic iron balance. Most sulfide microbially reduced in the water column reacts with reactive iron to precipitate iron sulfide or pyrite. If all of the microbially reduced sulfur were removed in the form of pyrite, the iron removal rate is stoichiometrically calculated to be as large as 1.6–4.8 × 108 t/yr, which is significantly larger than the removal flux of iron in the present ocean (Chester, 1990). Because pyrite is mainly formed at the oxic-anoxic interface in the Black Sea (Muramoto et al., 1991), reactive iron in the oxic part of the ocean (near the surface) or some marginal seas may have been reduced during the C-T event, if it was not sufficiently compensated for by additional inputs of iron from the anoxic deep water (Mottl and Holland, 1978) and by large GEOLOGY, June 1999
Figure 3. Kinetic model concerning sulfur cycle around Cenomanian-Turonian boundary. Gray bands indicate black shale. For time before 93.5 Ma, we assumed that relative fluxes of sulfur-removal pathways were set at 37% by hydrothermal processes, 40% by evaporite formation, and 23% by microbial sulfate reduction to keep mass and isotope balances (see also Holser et al. [1988] and Carpenter and Lohmann [1997]). During OAE-2 (from 93.5 to 92.7 Ma), only the flux due to microbial reduction is increased to fit δ34SSO4 record (from 16‰ to 22‰). Results applying three fractionation factors of microbial sulfate reduction (–20‰, –40‰, and –60‰) are shown.
hydrothermal activity due to the higher sea-floor spreading rate (Larson, 1991). Our model has some pitfalls: for example, the sulfate concentration in seawater can vary by a factor of three (Holland, 1972), and thus oceanic residence time of sulfur could also be a variable within a factor of three. The model described herein is, therefore, still primitive and requires more precise boundary conditions. Nevertheless, it strongly suggests that during the OAE-2, the sulfate reduction processes were significantly intensified, substantial amounts of sulfur and iron were removed from the ocean, GEOLOGY, June 1999
and an enormous amount of oxygen was released to the atmosphere. ACKNOWLEDGMENTS We thank H. Brumsack, I. Premoli Silva, R. Larson, T. Masuzawa, R. Tada, and E. Tajika for critical comments on the results, and N. Suits, H. Jenkyns, and L. Kump for helpful suggestions on the manuscript. This study is supported by a grant from the Japan Society for the Promotion of Science and the Program for Decoding Earth Evolution of Monbusho. REFERENCES CITED Arthur, M. A., and Premoli-Silva, I., 1982, Development of widespread organic carbon-rich strata in
the Mediterranean Tethys, in Schlanger, S. O., and Cita, M. B., eds., Nature and origin of Cretaceous carbon-rich facies: London, Academic Press, p. 7–54. Arthur, M. A., and Schlanger, S. O., 1979, Cretaceous “oceanic anoxic event” as causal factors in development of reef-reservoired giant oil fields: American Association of Petroleum Geologists Bulletin, v. 63, p. 870–885. Arthur, M. A., Dean, W. E., and Pratt, L. M., 1988, Geochemical and climatic effects of increased marine organic carbon burial at the Cenomanian/ Turonian boundary: Nature, v. 335, p. 714–717. Bralower, T. J., Fullagar, P. D., Paull, C. K., Dwyer, G. S., and Leckie, R. M., 1997, Mid-Cretaceous strontium-isotope stratigraphy of deep-sea sec537
tions: Geological Society of America Bulletin, v. 109, p. 1421–1442. Burdett, J. W., Arthur, M. A., and Richardson, M., 1989, A Neogene seawater sulfur isotope age curve from calcareous pelagic microfossils: Earth and Planetary Science Letters, v. 94, p. 189–198. Carpenter, S. J., and Lohmann, K. C., 1997, Carbon isotope ratios of Phanerozoic marine cements: Re-evaluating the global carbon and sulfur systems: Geochimica et Cosmochimica Acta, v. 61, p. 4831–4846. Chester, R., 1990, Marine chemistry: London, Unwin, 698 p. Claypool, G. E., Holser, W. T., Kaplan, I. R., Sakai, H., and Zak, I., 1980, The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation: Chemical Geology, v. 28, p. 199–260. Dean, W. E., Arthur, M. A., and Claypool, G. E., 1986, Depletion of 13C in Cretaceous marine organic matter: Source, diagenetic, or environmental signal?: Marine Geology, v. 70, p. 119–157. Goldhaber, M. B., and Kaplan, I. R., 1974, The sulfur cycle, in Goldberg, E. D., ed., The sea, Volume 5: New York, Wiley, p. 569–655. Gradstein, F. M., Agterberg, F. P., Ogg, J. G., Hardenbol, J., van Veen, P., Thierry, J., and Huang, Z., 1994, A Mesozoic time scale: Journal of Geophysical Research, v. 99, p. 24,051–24,074. Hayes, J. M., Popp, B. N., Takigiku, R., and Johnson, M. W., 1989, An isotopic study of biogeochemical relationships between carbonates and organic carbon in Greenhorn Formation: Geochimica et Cosmochimica Acta, v. 53, p. 2961–2972. Holland, H. D., 1972, The geologic history of seawater—An attempt to solve the problem: Geochimica et Cosmochimica Acta, v. 36, p. 637–652. Holland, H. D., 1973, Systematics of the isotopic composition of sulfur in the oceans during the Phanerozoic and its implications for atmospheric oxygen: Geochimica et Cosmochimica Acta, v. 37, p. 2605–2616. Holser, W. T., Schidlowski, M., Mackenzie, F. T., and Maynard, J. B., 1988, Biogeochemical cycles of carbon and sulfur, in Gregor, C. B., et al., eds., Chemical cycles in the evolution of the Earth: New York, Wiley, p. 105–173. Jenkyns, H. C., 1980, Cretaceous anoxic events: From continents to oceans: Geological Society [London] Journal, v. 137, p. 171–188.
538
Kajiwara, Y., Kaiho, K., and Ohkouchi, N., 1997, An invitation to the sulfur isotope study of marine carbonates: University of Tsukuba Institute of Geoscience, Annual Report, v. 23, p. 69–74. Kump, L. R., and Garrels, R. M., 1986, Modeling atmospheric O2 in the global sedimentary redox cycle: American Journal of Science, v. 286, p. 337–360. Larson, R. L., 1991, Latest pulse of Earth: Evidence for mid-Cretaceous superplume: Geology, v. 19, p. 547–550. Mottl, M. J., and Holland, H. D., 1978, Chemical exchange during hydrothermal alteration of basalt by seawater: 1. Experimental results for major and minor components of seawater: Geochimica et Cosmochimica Acta, v. 42, p. 1103–1115. Muramoto, J. A., Honjo, S., Fry, B., Hay, B. J., Howarth, R. W., and Cisne, J. L., 1991, Sulfur, iron and organic carbon fluxes in the Black Sea: Sulfur isotopic evidence for origin of sulfur fluxes: DeepSea Research, v. 38, p. S1151–S1187. Ohkouchi, N., Kawamura, K., Wada, E., and Taira, A., 1997, High abundances of hopanols and hopanoic acids in Cretaceous black shale: Ancient Biomolecules, v. 1, p. 183–192. Pedersen, T. F., and Calvert, S. E., 1990, Anoxia vs. productivity: What controls the formation of organic-carbon-rich sediments and sedimentary rocks?: American Association of Petroleum Geologists Bulletin, v. 74, p. 454–466. Pingitore, N. E., Meitzner, G., and Lowe, K. M., 1995, Identification of sulfate in natural carbonates by X-ray absorption spectroscopy: Geochimica et Cosmochimica Acta, v. 59, p. 2477–2483. Premoli Silva, I., Paggi, L., and Monechi, S., 1977, Cretaceous through Paleocene biostratigraphy of the pelagic sequence at Gubbio: Societá Geologica Italiana, Memorie, v. 15, p. 21–32. Robinson, B. R., and Kusakabe, M., 1975, Quantitative preparation of sulfur dioxide, for 34S/ 32S analyses from sulfides by combustion with cuprous oxide: Analytical Chemistry, v. 47, p. 1179–1181.
Printed in U.S.A.
Sasaki, A., Arikawa, Y., and Folinsbee, R. E., 1979, Kiba reagent method of sulfur extraction applied to isotopic work: Geological Society of Japan, Bulletin, v. 30, p. 241–245. Schidlowski, M., Hayes, J. M., and Kaplan, I. R., 1983, Isotopic inferences of ancient biochemistries: Carbon, sulfur, hydrogen, and nitrogen, in Schopf, J. W., ed., Earth’s earliest biosphere: Its origin and evolution: Princeton, New Jersey, Princeton University Press, p. 149–186. Schlanger, S. O., and Jenkyns, H. C., 1976, Cretaceous oceanic anoxic events: Causes and consequences: Geologie en Mijnbouw, v. 55, p. 179–184. Sweeny, R. E., and Kaplan, I. R., 1980, Stable isotope composition of dissolved sulfate and hydrogen sulfide in the Black Sea: Marine Chemistry, v. 9, p. 145–152. Takano, B., Asano,Y., and Watanuki, K., 1980, Characterization of sulfate in travertine: Contributions to Mineralogy and Petrology, v. 72, p. 197–203. Thiede, J., Dean, W. E., and Claypool, G. E., 1982, Oxygen-deficient depositional paleoenvironments in the mid-Cretaceous tropical and subtropical central Pacific Ocean, in Schlanger, S. O., and Cita, M. B., eds., Nature and origin of Cretaceous carbon-rich facies: London, Academic Press, p. 79–100. Vogt, P. R., 1989, Volcanogenic upwelling of anoxic, nutrient-rich water: A possible factor in carbonate-bank/reef demise and benthic faunal extinctions?: Geological Society of America Bulletin, v. 101, p. 1225–1245. Manuscript received November 2, 1998 Revised manuscript received March 8, 1999 Manuscript accepted March 15, 1999
GEOLOGY, June 1999
