M.R. Saltzman and E. Thomas Chapter 11 Carbon Isotope Stratigraphy Abstract: Variations in the 13 C/ 12 C value of total dissolved inorganic carbon (DIC) in the world’s oceans through time have been documented through stratigraphic study of marine carbonate rocks (d 13 C carb ). This variation has been used to date and correlate sediments. The stratigraphic record of carbon isotopes is complex because the main process frac- tionating 12 C from 13 C is photosynthesis, with organic matter depleted in the heavy isotope ( 13 C). The carbon isotope record (on the geological time scales considered here) is to a large extent defined by changes in the partitioning of carbon between organic carbon and carbonate, and therefore linked directly to the biosphere and the global carbon cycle. This chapter summarizes d 13 C carb variations through geologic time compiled from multiple literature sources. Materials analyzed for curve-construction differ between authors and between geological time periods, and one should carefully consider whether skeletal carbonate secreted by specific organisms or bulk carbonate has been used in evaluating or comparing carbon isotope stratigraphic records. Mid-Jurassic through Cenozoic curves have been mainly derived from pelagic carbonates, and exhibit low amplitude d 13 C carb variability (from 1 to þ4&) relative to curves for the earlier part of the record (from 3 to þ8 & for the Phanerozoic, from 15 to þ15& for the Proterozoic and Archean). The Mid-Jurassic and older curves are dominantly based on data from platform carbonates, which show greater variability and more spatial heterogeneity. The different character of carbon isotope curves derived from older platform carbonates as compared to younger pelagic records may reflect primary and/or diage- netic processes, difference in paleoenvironments, difference in calcifying organisms, or inherent changes in the global carbon cycle with geologic time and biotic evolution (e.g., changes in reservoir size). Chapter Outline 11.1. Principles of Carbon Isotope Stratigraphy 207 11.2. Spatial Heterogeneity of d 13 C of Dissolved Inorganic Carbon 209 11.3. Materials and Methods 210 11.3.1. Depositional Setting: Deep (Pelagic) Versus Shallow 210 11.3.2. Bulk Versus Component 217 11.3.3. Diagenesis 218 11.3.4. Global Versus Local Water Mass Signals 218 11.4. Correlation Potential and Excursions 219 11.5. Causes of Carbon Isotope Excursions 223 11.6. Conclusion 223 Acknowledgments 224 References 224 11.1. PRINCIPLES OF CARBON ISOTOPE STRATIGRAPHY The potential of marine carbonate d 13 C trends and excursions to date and correlate rocks relies on the fact that their 13 C/ 12 C ratios have varied over time, mainly as the result of partitioning of carbon between organic carbon and carbonate carbon reser- voirs in the lithosphere (e.g., Shackleton and Hall, 1984; Berner, 1990; Kump and Arthur, 1999; Falkowski, 2003; Sundquist and Visser, 2004). Precipitation of carbonates involves little carbon isotopic fractionation relative to dissolved inorganic carbon (DIC), and the d 13 C of carbonate is relatively insensitive to changes in temperature (about 0.035& per  C; Lynch-Stieglitz, 2003). Therefore the d 13 C of inorganically and biologically precipitated carbonate in the oceans is very close to that of the DIC in the oceans (Maslin and Swann, 2005), the largest reservoir in the recent ocean-atmosphere system (Figure 11.1). To explain changes in this isotopic signature we need to consider the global carbon cycle on different time scales (Falkowski, 2003; Sundquist and Visser, 2004). Carbon cycling between the ocean and the atmosphere occurs on time scales of <1000 years. At the present pH conditions of sea water (7.5e8.3), >90% of the carbon in the deep ocean is present as bicarbonate (HCO 3  ). The deep oceanic DIC reservoir is about 50 to 60 times as large as the atmospheric reservoir was in pre-industrial times (e.g., Ravizza and Zachos, 2003; Sundquist and Visser, 2004; Sarmiento and Gruber, 2006; Houghton, 2007). Carbon in the atmosphere is present as carbon dioxide (CO 2 ), whereas the lithosphere, which exchanges carbon with the The Geologic Time Scale 2012. DOI: 10.1016/B978-0-444-59425-9.00011-1 Copyright Ó 2012 Felix M. Gradstein, James G. Ogg, Mark Schmitz and Gabi Ogg. Published by Elsevier B.V. All rights reserved. 207