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 Dogie Creek, Montana, and Brownie Butte, Wyoming in the Western Interior of the United States. The boundary clays at both sites are not bounded by coals. Although coals consist mainly of organic matter derived from plant tissue, siliceous sedimentary rocks, such as shale and clay, may contain organic matter derived from microbiota as well as plants. Coals record δ13C values of plant-derived organic matter, reflecting the δ13C value of atmospheric CO2, whereas siliceous sedimentary rocks record the δ13C values of organic matter derived from plants and microbiota. The microbiota δ13C value reflects not only the δ13C 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 δ13C 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 δ13C values reflect the variation of δ13C of atmospheric CO2, which is in equilibrium with carbon isotopes at the ocean surface. Although a decrease in δ13C 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 δ13C-decrease at Dogie Creek is smaller than that at Brownie Butte and that for marine carbonate.About 2‰ decrease in δ13C of atmospheric CO2 was expected from the δ13C variation of marine carbonate at the K–T boundary. This δ13C-decrease of atmospheric CO2 should affect the δ13C values of organic matter derived from plant tissue. As such a decrease in δ13C value was not observed at Dogie Creek, a process that compensates the δ13C-decrease of atmospheric CO2 should be involved. For example, the enhanced contribution of 13C-enriched organic matter derived from algae in a high-productivity environment could be responsible. The δ13C 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 δ13C-decrease of atmospheric CO2 reflected a reduction of marine productivity, the compensation of the δ13C 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 δ13C 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 13C-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.