Carbon isotopic compositions of organic matter across continental Cretaceous–Tertiary (K–T) boundary sections: Implications for paleoenvironment after the K–T impact event

To assess the environmental perturbation induced by the impact event that marks the Cretaceous–Tertiary (K–T) boundary, concentrations and isotopic compositions of bulk organic carbon were determined in sedimentary rocks that span the terrestrial K–T boundary at DogieCreek,Montana, and Brownie Butte,Wyoming in theWestern Interior of the United States. The boundary clays at both sites are not bounded by coals. Although coals consistmainly of organicmatter derived fromplant tissue, siliceous sedimentary rocks, such as shale and clay, may contain organicmatter derived frommicrobiota as well as plants. Coals record δC values of plant-derived organic matter, reflecting the δC value of atmospheric CO2, whereas siliceous sedimentary rocks record the δ C values of organic matter derived from plants and microbiota. The microbiota δC value reflects not only the δC value of atmospheric CO2, but also biological productivity. Therefore, the siliceous rocks from these sites yields information that differs from that obtained previously from coal beds. Across the freshwater K–T boundary at Brownie Butte, the δC values decrease by 2.6‰ (from −26.15‰ below the boundary clay to −28.78‰ above the boundary clay), similar to the trend in carbonate at marine K–T sites. This means that the organic δC values reflect the variation of δC of atmospheric CO2, which is in equilibrium with carbon isotopes at the ocean surface. Although a decrease in δC values is observed across the K–T boundary at Dogie Creek (from −25.32‰ below the boundary clay to −26.11‰ above the boundary clay), the degree of δC-decrease at Dogie Creek is smaller than that at Brownie Butte and that for marine carbonate. About 2‰ decrease in δC of atmospheric CO2 was expected from the δ C variation of marine carbonate at the K–T boundary. This δC-decrease of atmospheric CO2 should affect the δ C values of organic matter derived from plant tissue. As such a decrease in δC valuewas not observed atDogieCreek, a process that compensates the δC-decrease of atmosphericCO2 should be involved. For example, the enhanced contribution of C-enriched organic matter derived from algae in a high-productivity environment could be responsible. The δC values of algal organic matter become higher than, and thus distinguishable from, those of plant organic matter in situations with high productivity, where dissolved HCO3 − becomes an important carbon source, as well as dissolved CO2. As the δ C-decrease of atmospheric CO2 reflected a reduction of marine productivity, the compensation of the δ C decrease by the enhanced activity of the terrestrial microbiota means that the microbiota at freshwater environment recovered more rapidly than those in the marine environment. A distinct positive δC excursion of 2‰ in the K–T boundary clays is superimposed on the overall decreasing trend at Dogie Creek; this coincides with an increase in the content of organic carbon. We conclude that the K–T boundary clays include ⁎ Corresponding author. Present address: Division of Integrative Environmental Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba City, Ibaraki 305-8572, Japan. Tel./fax: +81 29 853 4241. E-mail address: [email protected] (T. Maruoka). 1 Present address: 11186 Circle Drive, Golden, CO 80403, USA. 0012-821X/$ see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.10.028 227 T. Maruoka et al. / Earth and Planetary Science Letters 253 (2007) 226–238 C-enriched organic matter derived from highly productive algae. Such a high biological productivity was induced by phenomena resulting from the K–T impact, such as nitrogen fertilization and/or eutrophication induced by enhanced sulfide formation. The high productivity recorded in the K–T boundary clays means that the freshwater environments (in contrast to marine environments) recovered rapidly enough to almost immediately (within 10 yr) respond to the impact-related environmental perturbations. © 2006 Elsevier B.V. All rights reserved.