黔桂地区密西西比亚纪珊瑚礁的古生态学研究

Reefs are defined as essentially in place carbonate deposits created by sessile organisms, with lateral confinement and topographic relief. They are important ecosystems in the marine environments. Stromatoporoid-coral reef ecosystems collapsed and disappeared respectively during the Late Devonian Frasnian-Famennian (F-F) and the latest Famennian (Hangenberg) mass extinction events. Afterwards, the Mississippian was widely believed as a period dominated by microbial reefs, with rare metazoan reefs. In this project, systematic palaeontologic studies will be carried out on the reef builders and dwellers in the Mississippian coral reefs from the Guizhou and Guangxi regions in South China, which will uncover the biotic composition and its distribution characteristics in the reefs. Based on the researches on the lithologic and microfacies types, sedimentary structures and biotic taphonomic characteristics of the coral reefs, their sedimentary environments will be reconstructed. Furthermore, the palaeoecology of the coral reefs will be discussed, such as the relationships among the reef community successions, growth and demise processes and sedimentary environment variations. The age of the coral reefs will be constrained by the foraminiferal fossils from the reefs and their underlying and overlying strata, which is the basis for the comparison of the coeval coral reefs between South China and the other global areas in their palaeoecological characteristics including biotic composition, taphonomic characteristic, sedimentary environment, and growth and demise process. These results will provide new insights into the composition, recovery and evolutionary process of the Mississippian metazoan reefs in the aftermath of the disappearance of end-Devonian stromatoporoid-coral reefs.

生物礁是由原地固着生物的沉积作用形成、横向分布限制并具有正向隆起的碳酸盐建造，是海洋中重要的生态系统。晚泥盆世弗拉期与法门期之交（F-F）和法门末期（Hangenberg）生物灭绝事件分别导致了层孔虫-珊瑚礁生态系统的崩溃和消失，之后的密西西比亚纪被普遍认为以微生物礁为主，后生动物礁罕见。本项目将对华南黔桂地区密西西比亚纪珊瑚礁的造礁和附礁生物进行系统古生物学研究，揭示礁体的生物组成和分布特征；对珊瑚礁的岩石和微相类型、沉积结构和生物埋藏特征进行研究，恢复礁体的沉积环境；进而探讨礁体的群落演替、形成和消失过程及其与沉积环境变化的关系等古生态信息。根据珊瑚礁及其上下地层中的有孔虫化石确定礁体的时代，探讨华南与全球其它地区同时期珊瑚礁的生物组成、埋藏特征、沉积环境、形成和消失过程等古生态特征的异同性，为泥盆纪末期层孔虫-珊瑚礁消失之后，密西西比亚纪后生动物礁的组成、复苏演化过程等提供新认识。

后生动物礁具有极高的生物丰度和多样性，是海洋中重要的生态系统，可以指示海洋古生态变化。伴随晚泥盆世生物灭绝事件，显生宙最大的后生动物（层孔海绵-珊瑚）礁系统发生了崩溃和消失，之后的密西西比亚纪长期被认为以微生物礁系统主导，缺乏后生动物礁发育。本项目通过对广西甘垌子剖面和贵州雅水剖面密西西比亚纪珊瑚礁的系统古生物学和沉积学研究，发现甘垌子剖面发育丰富的珊瑚礁，大小为几米到几十米，具有明显的正向隆起；雅水剖面为层状珊瑚礁，厚度几十公分，宽度至少几十米，无明显的正向隆起。它们主要由丛状复体四射珊瑚格架岩组成，含少量横板珊瑚格架岩。甘垌子剖面造礁珊瑚包括复体四射珊瑚（双型珊瑚Diphyphyllum lateseptatum，丛管珊瑚Siphonodendron martini），横板珊瑚（多壁管珊瑚Multithecopora sp.和笛管珊瑚Syringopora sp.）；雅水剖面造礁珊瑚包括丛管珊瑚Siphonodendron pauciradiale和板柱珊瑚Stylostrotion petalaxoidea。附礁生物多样性高，包括海百合茎、腕足类、钙球、绿藻、苔藓虫、有孔虫、介形虫、腹足类和异珊瑚。岩相和微相研究显示，礁体中发育丰富的骨架岩，可以与礁翼的堆积灰岩、颗粒灰岩、泥粒灰岩等微相区分。礁体及其上下地层中生物和沉积组分的变化指示珊瑚礁的发育与相对海平面变化密切相关，形成于相对海平面升高时期。利用有孔虫确定华南板块珊瑚礁的时代为密西西比亚纪维宪期晚期，它们与全球其它板块同时期珊瑚礁具有相似的生物和沉积组成。结合新发现和已发表的生物礁数据，重建了晚泥盆世-密西西比亚纪生物礁演化过程，揭示了维宪期晚期全球存在珊瑚礁系统繁盛，伴随海洋生物多样性的显著增加，这指示该时期出现稳定的海洋生态系统。利用雅水和甘垌子剖面腕足类壳体氧同位素，重建了密西西比亚纪中-晚期古海水温度变化曲线，发现在维宪期晚期（Asbian-Brigantian之交）腕足类壳体氧同位素存在一次显著正偏，幅度约2‰，其指示该时期古海水温度降低约5℃。这次全球气候变冷可能代表了晚古生代大冰期主幕的开始，是导致维宪期晚期珊瑚礁丰度和生物多样性减少的主要原因。