Carbon isotopic compositions of organic matter across continental Cretaceous–Tertiary (K–T) boundary sections: Implications for paleoenvironment after the K–T impact event
Carbon isotopic compositions of organic matter across continental Cretaceous–Tertiary (K–T) boundary sections: Implications for paleoenvironment after the K–T impact event
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DOI：
10.1016/j.epsl.2006.10.028
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发表时间：
2007-01
影响因子：
5.3
通讯作者：
T. Maruoka;C. Koeberl;B. Bohor
中科院分区：
文献类型：
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作者：
T. Maruoka;C. Koeberl;B. Bohor
为了评估标志白垩纪-第三纪（K-T）边界的撞击事件引起的环境扰动，在美国西部内陆蒙大拿州Dogie Creek和怀俄明州Brownie Butte跨越陆地K-T边界的沉积岩中测定了大块有机碳的浓度和同位素组成。这两个地点的边界粘土均未被煤所包围。虽然煤主要由来自植物组织的有机质组成，但硅质沉积岩，如页岩和粘土，也可能含有来自微生物群和植物的有机质。煤记录了植物源有机质的δ13C值，反映了大气CO2的δ13C值，而硅质沉积岩记录了植物和微生物源有机质的δ13C值。微生物群δ13C值不仅反映了大气CO2的δ13C值，还反映了生物生产力。因此，从这些地点的硅质岩石中获得的信息与以前从煤层中获得的信息不同。在Brownie Butte的淡水K-T边界上，δ13C值下降了2.6‰（从边界粘土以下的- 26.15‰下降到边界粘土以上的- 28.78‰），与海相碳酸盐中K-T的变化趋势相似。这意味着有机δ13C值反映了大气CO2 δ13C的变化，与海洋表面碳同位素平衡。虽然在Dogie Creek的K-T边界处δ13C值下降（从边界粘土以下的- 25.32‰下降到边界粘土以上的- 26.11‰），但Dogie Creek的δ13C下降程度小于Brownie Butte和海相碳酸盐岩。从K-T边界海相碳酸盐岩δ13C变化预测大气co2 δ13C降低约2‰。大气co2的δ13C下降会影响植物组织有机质的δ13C值。由于Dogie Creek没有观测到δ13C值的下降，因此可能存在一个补偿大气co2 δ13C下降的过程。例如，在高生产力环境中，来自藻类的富含13c的有机物的贡献可能是负责任的。在高生产力条件下，藻类有机质的δ13C值高于植物有机质的δ13C值，溶解的HCO3−和溶解的CO2成为重要的碳源，从而与植物有机质的δ13C值有所区别。大气co2 δ13C的下降反映了海洋生产力的下降，陆地微生物群活动的增强补偿了δ13C的下降，这意味着淡水环境的微生物群比海洋环境的微生物群恢复得更快。在Dogie Creek地区，K-T边界粘土的δ13C明显的正偏移（2‰）叠加在整体下降趋势上；这与有机碳含量的增加是一致的。我们得出结论，K-T边界粘土包括来自高产藻类的富13c有机质。如此高的生物生产力是由K-T影响引起的现象引起的，例如氮肥和/或硫化物形成增强引起的富营养化。在K-T边界粘土中记录的高生产力意味着淡水环境（与海洋环境相反）恢复得足够快，几乎可以立即（在10年内）对与冲击相关的环境扰动做出反应。
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 CO2was expected from the δ13C variation of marine carbonate at the K–T boundary. This δ13C-decrease of atmospheric CO2should 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 CO2should be involved. For example, the enhanced contribution of13C-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 CO2reflected 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 include13C-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.