浙赣交界地区晚奥陶世生物礁群落生态梯度分析及其古气候意义

The Upper Ordovician (upper Katian) Xiazhen and Sanqushan formations in the Zhejiang-Jiangxi border region of southeastern China contain abundant and diverse reefs. In the nearshore areas, coral-stromatoporoid reefs are exposed in the Xiazhen Formation. In contrast, huge microbial reefs are dominant in the Sanqushan Formation of the offshore areas. The main features of these reefs have been well described, but little is known about the causes of the onshore–offshore transition in biotic composition. And the response of reefal ecosystems to the late Katian (preglacial period) Boda Event is still poorly understood. More importantly, descriptive studies could not concretely reveal species-environmental relationships in reefs and quantitative investigation will be necessary. The present study focuses on the quantitative palaeoecology of the late Katian reefs of the Zhejiang-Jiangxi border region by combining facies analysis and gradient analysis. Based on the comprehensive data, we try to reconstruct the reef communities and discuss their palaeoenvironmental implications. We attempt to test if bathymetry and turbidity are the key factors of the onshore–offshore transition in reefs. By comparing the data from the Zhe-Gan Platform to that from the late Katian reefs outside South China, we try to figure out how the reefs responded to the climate changes, to test the palaeoclimatic models and to address key questions and hypotheses of the pre-Hirnantian Boda Event.

华南浙赣交界地区上奥陶统（凯迪阶上部）发育了同期异相的下镇组和三衢山组两套含礁地层。近岸相下镇组保存了典型的珊瑚-层孔海绵礁，而远岸相三衢山组则以巨厚的微生物礁为特征。然而，上述从近岸到远岸礁体中生物组成差异的原因及控制因素仍不明确，研究区礁系统对凯迪晚期Boda事件的响应方式也有待进一步研究。传统的定性化研究难以有效揭示化石礁群落-环境因子间的协同关系，有必要采用定量化的手段进行探索。本项目以该区凯迪晚期生物礁为研究对象，结合沉积相和生态梯度分析，开展定量古生态学研究，恢复古环境和礁群落面貌，侧重解析水深和浑浊度梯度等因素对礁群落结构及其分布的控制力度。同时，通过比较华南案例与其他块体同期礁体的异同，验证关键地史时期的古气候模型，探讨礁生态系统对气候变化的响应模式，进而深化对赫南特冰期前Boda事件的过程及其实质的认识。

在华南浙赣交界地区，上奥陶统凯迪阶的下镇组和三衢山组两套含礁地层中，从近岸到远岸生物礁组成差异的原因及控制因素并不明确，研究区礁生态系统对晚奥陶全球气候的响应方式亦有待研究。本项目以该区凯迪晚期生物礁为主要研究对象，纵向对比华南早奥陶世和早志留世的案例，探讨古气候和古地理对晚奥陶世礁系统的影响。根据牙形石组合和碳同位素曲线，结合沉积学证据（例如古喀斯特等），证明近岸相的下镇组记录了晚奥陶世凯迪中期的沉积间断，是我国华南块体对这一短暂冰期的首次发现。显示在赫南特冰期之前可能存在不止一次中等规模的冰期，而下镇组中的生物礁则是在间冰期的背景下发育的。在相对远岸相的三衢山组中发育了含串管海绵和层孔海绵的微生物礁。虽然毗邻古陆，但得益于孤立台地的特点，浙赣台地仍然发育了自养型生物（钙质微生物和钙藻）为主导的礁群落。与早奥陶世相比，凯迪阶的生物礁中后生动物造礁作用显著增强，表明随着温室气候的解除，奥陶纪生物辐射事件对礁生态系统造成了革命性的影响。