Confiabilidad y validez

The δ34_{S value of sulphate in marine waters has fluctuated throughout Earth’s}

geological history. At present, the oceans contain an estimated 130,000 metric tonnes of sulphur represented by sulphate in solution (Faure and Mensing, 2005). Input of sulphur into the ocean is typically via the discharge of sulphate ions by rivers where it originates

as a weathering product. It can also enter the system from hydrothermal fluids discharged into the ocean at divergent plate boundaries or through the direct contact between volcanically heated sea waters and oceanic crust (Faure and Mensing, 2005). Sulphur leaves the aqueous marine system via precipitation either as evaporites

(anhydrite or gypsum) or as metal sulphides following the reduction of sulphate to H2S

(Faure, 1998). Under ideal circumstances, evaporites show a minimal enrichment in 34S of approximately 1.65‰ when compared to marine sulphate levels (Faure, 1998). This negligible difference allows evaporites to be used as proxies for paleo marine sulphate levels and therefore the sulphur isotope curve through the Phanerozoic can be mapped.

The Phanerozoic sulphur isotope curve shows considerable variation with

maximum δ34S values of +30‰ in the Cambrian, dropping to +10‰ in the Permian, then increasing nonlinearly throughout the Mesozoic to the present value of ~20‰ (Faure, 1998; Faure and Mensing, 2005; Hoefs, 2009). Fluctuations in the curve are interpreted as reflecting changes in the global sulphur cycle, although the specific causes of such fluctuations are under debate. There are challenges when trying to refine the sulphur isotope curve and to extend it into the Precambrian. Evaporites are not continuously deposited, often contain gaps, and typically have poor time resolution (Hoefs, 2009). It has been proposed that using isotopic signatures from carbonates or marine barite would produce a more accurate and complete paleo sulphur isotope curve (Burdett et al., 1989; Kampschulte and Strauss, 2004). Work is currently underway to determine when sulphur reducing micro-organisms evolved during the Precambrian using δ34S values; however inconsistent results and highly metamorphosed deposits have yet to yield a reliable age (Kampschulte and Strauss, 2004).

References

Allègre, C. J., 2008. Isotope geology. Cambridge University Press, Cambridge.

Beaudoin, G., Taylor, B.E., Rumble, D. III., and Thiemens, M., 1994. Variations in the sulfur isotope composition of troilite from the Canon Diablo iron meteorite. Geochimica et Cosmochimica Acta 58, 4253-4255.

Berglund, M., and Wieser, M.E., 2011. Isotopic compositions of the elements 2009 (IUPAC Technical Report). Pure and Applied Chemistry 83, 397-410.

Brunner, B., Bernasconi, S. M., Kleikemper, J., and Schroth, M. H., 2005. A model for

oxygen and sulfur isotope fractionation in sulfate during bacterial sulfate

reduction processes. Geochimica et Cosmochimica Acta 69, 4773-4785.

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 94, 189-198.

Canfield, D.E., 2001(a). Biogeochemistry of sulfur isotopes. Reviews in Mineralogy and Geochemistry 43, 607-636.

Canfield, D.E., 2001(b). Isotope fractionation by natural populations of sulfate-reducing bacteria. Geochimica et Cosmochimica Acta 65, 1117-1124.

Canfield, D.E., and Thamdrup, B., 1994. The production of 34_{S-depleted sulfide during}

bacterial disproportionation of elemental sulfur. Science 266, 1973-1975.

Faure, G., 1998. Principles and applications of geochemistry; a comprehensive textbook for geology students; Second Edition. Prentice Hill, Upper Saddle River, New Jersey.

Hoboken, New Jersey.

Harrison, A.G., and Thode, H.G., 1957. The kinetic isotope effect in the chemical reduction of sulphate. Transactions of the Faraday Society 53, 1648-1651.

Harrison, A.G., and Thode, H.G., 1958. Mechanism of the bacterial reduction of sulphate from isotope fractionation studies. Transactions of the Faraday Society 54, 84-92. Hoefs, J., 2009. Geochemistry; Sixth Edition. Springer-Verlag, Berlin.

Jørgensen, B.B., Böttcher, M.E., Lüschen, H., Neretin, L.N., and Volkov, I.I., 2004. Anaerobic methane oxidation and a deep H2S sink generate isotopically heavy

sulfides in Black Sea sediments. Geochimica et Cosmochimica Acta 68, 2095- 2118.

Kampschulte, A., and Strauss, H., 2004. The sulfur isotopic evolution of Phanerozoic seawater based on the analysis of structurally substituted sulfate in carbonates. Chemical Geology 204, 255-286.

Kaplan, I. R., and Rittenberg, S. C., 1964. Microbiological fractionation of sulphur

isotopes. Journal of General Microbiology 34, 195-212.

Kemp, A. L. W., and Thode, H. G., 1968. The mechanism of the bacterial reduction of

sulphate and of sulphite from isotope fractionation studies. Geochimica et

Cosmochimica Acta 32, 71-91.

McCready, R. G. L., 1975. Sulphur isotope fractionation by Desulfovibrio and

Desulfotomaculum species. Geochimica et Cosmochimica Acta 39, 1395-1401.

Rudnicki, M. D., Elderfield, H., and Spiro, B., 2001. Fractionation of sulfur isotopes

during bacterial sulfate reduction in deep ocean sediments at elevated

White, W.M., 2013. Geochemistry; First Edition. John Wiley & Sons Ltd., Chichester, West Sussex.

Wortmann, U.G., Bernasconi, S.M., and Böttcher, M.E., 2001. Hypersulfidic deep biosphere indicates extreme sulfur isotope fractionation during single-step microbial sulfate reduction. Geology 29, 647-650.

Chapter 6

6

The paleoenvironment of Late Devonian eastern North

America as interpreted from the lithological,

paleontological, and geochemical analyses of the Kettle

Point Formation, southwestern Ontario, Canada

Introduction

Despite their importance as hydrocarbon reservoir and source rocks, organic-rich, siliciclastic mudrocks, particularly black shales, have been understudied due to their apparent homogeneous nature and lack of modern depositional analogues. Detailed recent studies have revealed that black shales are not as monotonous as once thought, nor are they necessarily indicative of quiet, deep-water environments (i.e. Schieber, 1994; 1998). Organic-rich deposits are, in fact, quite complex and can form in a wide range of depositional settings from deep to shallow water raising questions about both the nature and scale of causative factors of widespread basinal anoxia as recorded by black shales (Schlanger and Jenkyns, 1976; Schieber, 1994; 1998; Ozaki et al., 2011).

Black shales of Paleozoic age are relatively common and geographically

widespread (Negri et al., 2009a; Jenkyns, 2010), however the lack of comparable modern depositional systems has resulted in an incomplete understanding of both the processes behind the formation of these organic-rich sediments and the environments in which they were deposited. Prominent black shale successions, such as the Upper Devonian black shales of eastern North America, have been noted to be temporally associated with major geological events of regional to global scale (e.g. orogenies and mass extinctions). More

specifically the timing of Devonian black shale deposition was coincident with the Acadian orogeny on the eastern margin of North America, and the Late Devonian mass extinction event. Interpreting the environments in which black shales were deposited and how they changed through time provides insights into biosphere-atmosphere-

hydrosphere-geosphere interactions during these highly dynamic, and sometimes

enigmatic, episodes in Earth’s history. This allows for the impact of global and regional factors on sedimentation to be assessed.

To decipher the paleoenvironment of southwestern Ontario during the Late Devonian, and by proxy, the paleoenvironment of eastern North America, this study investigates one of the thickest and most laterally continuous black shale deposits in Ontario; the Kettle Point Formation. The Upper Devonian Kettle Point Formation

features a distinct interbedding of organic-rich and organic-poor lithofacies. In this study, these lithofacies are referred to as “black shales” and “greyish green mudstones”

respectively. To date, little is understood about the relationship between the organic-rich and organic-poor units, or their origins, in particular the source of the greyish green mudstones, in the Kettle Point Formation and in coeval shale deposits. Additionally, the factors that resulted in the abrupt transitions between the organic-rich and organic-poor layers remains unexplained. This study aims to interpret the depositional environment in which the Kettle Point Formation was formed using a combination of lithological,

paleontological, and geochemical (sulphur isotope) analyses. In addition, this study will identify possible factors that influenced the deposition of black shales versus greyish green mudstones in southwestern Ontario and throughout eastern North America.

Geological history of the Kettle Point Formation

The Upper Devonian Kettle Point Formation is partially exposed in a few

outcrops in its type area (Kettle Point, Ontario, Canada), therefore the formation has been studied primarily from subsurface cores in southwestern Ontario (e.g., Russell, 1985). The present outcrop/subcrop belt of the Kettle Point Formation is largely confined to the Chatham Sag, a saddle-like basement depression in the Algonquin-Findlay Arch system which is paleogeographically situated between the intracratonic Michigan Basin and the Appalachian Foreland Basin. The formation was deposited during the Late Devonian (Frasnian and Famennian) in an epeiric sea that covered eastern North America making the Kettle Point stratigraphically equivalent to the more extensive black shale units of the eastern United States, including the Ohio, New Albany, Chattanooga, and Antrim Shales. The formation is also syndepositional with other organic-rich, fine-grained deposits from the Hudson Bay Platform (the Long Rapids Formation) (Figure 2.10).

Previous research on the Kettle Point Formation has focused on its mineralogy (Delitala, 1984; Armstrong, 1986), petrology (Armstrong, 1986), and viability as a hydrocarbon source rock (e.g. Russell and Barker, 1983; Russell, 1985; Hamblin, 2006; 2010; Béland Otis, 2013). Few studies have concentrated on identifying the lithological and paleontological components of the Kettle Point Formation, determining how they change throughout the interval, or relating variations in the components to local and/or global factors to resolve the depositional conditions recorded in the formation. The paleogeographic position of the Kettle Point Formation, between the Antrim and Ohio shales, suggests that whatever factors are identified as influencing the deposition of

organic-rich versus organic-poor sediment in southwestern Ontario can be extrapolated to the adjacent shales, and potentially to the other syndepositional, interconnected deposits in eastern North America.

Materials and methods

A combination of lithological (core and thin section analysis), paleontological, and geochemical techniques were employed to resolve the depositional environment of the Kettle Point Formation.