Chinese Science Bulletin © 2008 SCIENCE IN CHINA PRESS Springer Geochemical environmental changes and dinosaur extinction during the Cretaceous-Paleogene (K/T) transition in the Nanxiong Basin, South China: Evidence from dinosaur eggshells ZHAO ZiKui1†, MAO XueYing2, CHAI ZhiFang2, YANG GaoChuang2, ZHANG FuCheng1 & YAN Zheng3 1 Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China; 2 Institute of High Energy Physics and Laboratory of Nuclear Analytical Techniques, Chinese Academy of Sciences, Beijing 100080, China; 3 Institute of Geology, State Seismological Bureau, Beijing 100029, China The complex patterns of trace elements including Ir and isotope distributions in the three K/T sections of the Nanxiong Basin prove the existence of two environmental events in the latest Cretaceous and earliest Paleocene. The first geochemical environmental event occurred at about 2 Ma prior to the K/T boundary interval, where the dinosaur diversity was hardly reduced, except that a number of patho- logical eggshells appeared. The second one was larger and occurred just at and near the Creta- ceous-Paleogene (K/T) boundary. The extinction of the dinosaurs spread out within 250 ka with major extinction beginning at the boundary interval. This is even later than their extinction in Montana, North America and in India. The cause of the dinosaur extinction may be the result of a complex multiple events brought about by the coincidence of global environment change marked by multiple Ir and δ 18O anomalies, and environmental poisoning characterized by other trace elements derived from the local source. Successive short- and long-term conditions of geochemically induced environmental stress negatively affected the reproductive process and thus contributed to the extinction of the dinosaurs. Nanxiong Basin of Guangdong Province, Cretaceous-Paleogene (K/T) boundary, Ir anomaly, trace element, stable isotope, dinosaur eggshell, dinosaur extinction One of the most striking events in the Mesozoic era was Province are a relatively continuous sequence containing the almost complete extinction of the dinosaurs at the end a record of dinosaur egg fauna spanning the K/T bound- of the Cretaceous. The observation of a widespread Ir ary and Paleocene mammals, and seem to provide some anomaly at the K/T boundary led by Alvarez et al.[1―3] to direct evidence for interpreting the dinosaur extinction. propose that the extinction event at the boundary results Previously published data[10―16] from the CGY-CGD and from the impact of an extraterrestrial body on earth. CGT-CGF sections (Figure 1) showed that the geo- However, it is difficult to find strong support for this hy- chemical environmental changes and the dinosaur ex- pothesis, because of the paucity of suitable stratigraphic tinction in South China may have been a rather long- sections and the comparative scarcity of fossil material. Virtually all the evidence or debates that have been drawn Received February 20, 2008; accepted October 30, 2008; published online December 14, 2008 about the final dinosaur extinction derive from a few sec- doi: 10.1007/s11434-008-0565-1 tions in the North American Western Interior[4―9]. †Corresponding author (email: [email protected]) Supported by National Natural Science Foundation of China (Grant No. 40472018) The “red beds” in the Nanxiong Basin of Guangdong and the Chinese Academy of Sciences (Grant No. 21039751) www.scichina.com | csb.scichina.com | www.springerlink.com Chinese Science Bulletin | March 2009 | vol. 54 | no. 5 | 806-815 duration process rather than an instantaneous event that Formation, from which about 10 clutches of eggs have only lasted a few years or centuries as is often portrayed been collected by the local museums (the Nanxiong ARTICLES in the asteroid hypothesis. Museum and the Shaoguan Museum) and the Shanghai Museum of Natural History from 1972 to 1994. The complete clutches preserved in situ show no evidence of transport. Excellent preservation of the clutch geometry indicates that the egg laying and sediment formation were nearly synchronous. Many eggshell fragments were found in heaps in this section, and may represent clutches destroyed in situ or nearby. It suggests that the sediments with abundant eggshell fragments were clearly not reworked. Because palynomorphs were scarcer than expected, the exact location of the K/T boundary interval has not yet been determined in this section[15], but it should be present at CGN 220―240 m, Figure 1 Locations of the studied K/T boundary sections in the as discussed later. Nanxiong Basin. Fifteen eggshell samples, belonging to Macroolithus To obtain a better idea on how the cause and timing of yaotunensis, collected at 15 levels from this section by [10,15] the dinosaur extinction took place, we have investigated the Sino-German team in 1984 have been analyzed trace elements including Ir, stable isotope composition, by radiochemical neutron activation analysis (RNAA) and eggshell structures in a series of dinosaur eggshells for their iridium concentration, and by instrumental neu- from the third K/T section (the CGN section) in this ba- tron activation analysis (INAA) for other trace elements; sin. besides, another 15 eggshell samples collected at the The purpose of the present paper is to establish the same levels of this section were used for stable-isotope environmental change pattern across the K/T boundary analysis. The eggshell samples for the chemical analysis and the geochemical environmental stress played a role have been examined using a light microscope and/or in dinosaur extinction, based on the geochemical signals SEM, and display a well-preserved microstructure with GEOLOGY (e.g. trace elements including Ir, stable carbon- and primary calcite growth and little or no recrystallization. oxygen-isotope composition), histo-structures of dino- The eggshell samples were then abraded on the outer saur eggshells with other data regarding stratigraphy and inner surfaces in order to remove any contamination taken from the above-mentioned three sections (CGY- by the surrounding sediments. CGD, CGT-CGF and CGN sections). Theoretically, Ir is nonexistent in eggshell and/or bone because this element is not vital to life. Eggshell 1 Element and isotope distribution in samples of living ostrich (Struthio camelus) from Bei- jing Zoo, wild Chinese alligator (Alligator sinensis) dinosaur eggshells from the CGN Section from Anhui Province, and chickens (Gallus gallus) [12,17] The CGN Section is situated south of the Nanxiong showed no detectable Ir with limits of <6―<10 ppt . county town (Figure 1). Lithologically, the section is The results of Ir abundances and the stable carbon- essentially similar to both the CGY-CGD Section and and oxygen-isotope composition in the eggshell samples the CGT-CGF Section. However, the Pingling Formation from the CGN Section are given in Table 1. Four here attains a thickness of about 270 m, and its upper- Ir-bearing levels have been identified. The highest Ir most part and the overlying strata, the Shanghu Forma- abundance occurs in eggshell sample CGN 912 from tion, are not exposed because of the town buildings in 215.5 m in the CGN Section, and reaches 236.8 ppt this area. (normalized to Ca) over a background level of <10 ppt. According to the field observation, complete eggs in The three other Ir peaks (of 42.7 ppt, 53.3 ppt and 179.7 nest and eggshell fragments are more frequent and ppt) were detected in the eggshell samples CGN 909, widespread in the sedimentary sequence of the Pingling CGN 903 and CGN 901 from 202―203 m, 85.5 m and ZHAO ZiKui et al. Chinese Science Bulletin | March 2009 | vol. 54 | no. 5 | 806-815 807 Table 1 Iridium concentration and stable-isotope composition in the dinosaur eggshell samples of Macroolithus yaotunensis from the CGN Section of the Pingling Formation Ir concentration Ir concentration normalized to Sample number Depth (m) Ca (%) δ 13C (‰, PDB) δ 18O (‰, PDB) (ppt, 10−12g/g) Ca (ppt, 10−12g/g) CGN 915 235―235.5 29.7 39.5 28.0 −10.93 +5.50 CGN 914 231.5―232 29.4 39.6 27.6 −10.27 −3.29 CGN 913 223.5―225 <16.3 9.25 −10.18 −3.85 CGN 912 215.5 268 42.1 236.8 −10.85 +0.36 CGN 911 213.5 <18.5 41.4 −10.09 −1.63 CGN 910 208 8.91 41.4 8.1 −9.62 +1.45 CGN 909 202―203 48.4 42.2 42.7 −9.90 −2.19 CGN 908 198―199.5 23 42.8 20.0 −12.01 −6.75 CGN 907 186 25.1 40.5 23.1 −11.13 −2.31 CGN 906 176.5 26.5 40 24.6 −10.55 −3.26 CGN 905 110―112 29.3 42.5 25.6 −10.77 −2.20 CGN 904 104.5―106 <11.6 41.9 −10.46 −3.36 CGN 903 85.5 60.6 42.3 53.3 −9.54 −4.54 CGN 902 76.5 <20.9 41.8 −9.09 +1.11 CGN 901 44 185 38.3 179.7 −9.23 −0.97 44 m of the section, respectively. The distribution of oospecies: Macroolithus yaotunensis, Macroolithus ru- some other trace elements (As, Br, K, La, Na, Au, U, Yb, gustus, Elongatoolithus andrewsi, Elongatoolithus elon- Fe, Co, Sc and Sr) shows (Figure 2(c)) that most of them gatus, Elongatoolithus oosp., Apheloolithus shuinanen- are distinctly the most abundant in eggshell sample sis, Nanshiungoolithus chuetienensis and Shixingoolithus CGN 913 from 223.5―225 m of the same section, ex- erbeni. cepting Au, U and Fe whose maximum levels occur in Normal eggshells of different oospecies have each the eggshell sample CGN 901 from 44 m. structural pattern and a normal range of shell thickness.