华南地区二叠纪硅质岩序列天文调谐及其古气候意义

Chert, a type of silica-rich chemical sedimentary rock, is simply composed of micro- and crypto-crystalline quartz, which records plenty of origin information of sedimentary environment. In recent years, cyclostratigraphy studies of Mesozoic and Paleozoic radiolarian chert rhythmites have focused on chert-mudstone couplet series and 1-0 rank series. Astronomical tuning of chert-mudstone series have identified Milankovitch cycles and unveiled orbital forcing of paleoclimate changes, which by far is not well known in chert-rich strata. However, there are only few simple paleoclimate proxies of lithological variations in chert rhythmites. For couplet series and 1-0 series, the paleoclimate information and cyclic signals within single layers have been abandoned. In that case, the tuning results of those proxies have imposed restrictions on deep analysis and interpretation of geological time and climate change in chert sequence. Thus, it is very important to select optimal chert outcrops to explore and evaluate paleoclimate proxies for further study of chert cyclostratigraphy. It is well known that Permian chert sequences are widespread across South China, characterized by centimeter-scale chert-mudstone (chert-limestone) cycles. This project is designed to develop several kinds of paleoclimate proxies in Permian cherts from South China, including the cyclic variations of continuous lithological, magnetic and lamina proxies throughout chert outcrops. Based on tuning results of newly developed paleoclimate proxies, the project aims to establish high resolution chert astronomical time scale, and give insights into time scale and paleoclimate change from Late Paleozoic Ice Age siliceous organism productivity, which provide a chert perspective for cyclostratigraphy and deep-time paleoclimate research.

硅质岩是一类富硅化学沉积岩，主要由微晶-隐晶石英组成，其矿物组成简单却保存多种成因信息。近年来，硅质岩韵律层已开展岩性旋回分析，硅质岩-泥岩的层偶序列和1-0等级序列调谐出了米兰科维奇旋回信号，揭示了少受关注的硅质岩地层蕴含的地球轨道驱动特征。但是，硅质岩韵律层岩性变化作为古气候指标相对单调，层偶和1-0序列对古气候信息进行了简化，导致单个地层中的旋回信号被遗漏，从而制约了硅质岩中地质时间和气候变化的深入解读。优选硅质岩剖面广泛提取古气候指标，是硅质岩旋回地层学进一步研究的关键前提。华南二叠纪硅质岩分布广泛，普遍发育硅质岩-泥岩(灰岩)韵律层。本项目拟在华南二叠纪硅质岩中开发多种古气候指标，查明剖面上岩性、磁性、纹层等指标的旋回变化特征；通过天文调谐，旨在建立高精度硅质岩天文年代标尺，解读晚古生代冰期硅质生物繁盛所记录的时间尺度和气候演化，为旋回地层学和深时古气候研究提供硅质岩视角。

硅质岩韵律层的旋回地层学分析中，硅质岩-泥岩岩性旋回的层偶序列和1-0箱状序列调谐出了米兰科维奇旋回信号，揭示了少受关注的硅质岩地层蕴含的地球轨道驱动特征。本项目通过对华南巢湖地区中二叠统孤峰组硅质岩-泥岩的韵律层开展了系统研究，重建了硅质岩-泥岩序列中4种岩性古气候指标：层偶序列、箱状序列、三角序列和波状序列。同时针对二叠纪尚未有理论天文参数模型，利用主要的270Ma天文参数（即斜率和岁差数值），重建了中二叠世270-264 Ma的6百万年斜率-岁差的时间序列天文模型。三角序列和波状序列在硅质岩-泥岩旋回地层学研究中，能够突出斜率和岁差范围内的米兰科维奇信号，减小低频和高频信号能量，并且在硅质岩-泥岩旋回地层中显示了箱状序列并未调谐出来的斜率和岁差随时间交替发育的特征。这种斜率和岁差交替驱动的天文胁迫模式，暗示轨道尺度气候演变模式来源于两方面：一方面受岁差驱动季风气候控制的上升流发育，促使硅质生物周期性繁盛，另一方面是斜率驱动的南半球冈瓦纳大陆冰川消融，导致海平面升降则控制陆源泥质的周期性输入。应用三角序列的调谐结果，结合最新同位素地质年代学数据，重新标定了孤峰组沉积年限，从270.0 Ma 到264.642 Ma，持续了5.4 Myr。同时，本项目对二叠纪不同成因类型的硅质岩进行地球化学研究，选取了华北地区早二叠世结核状硅质岩进行成因分析，通过地层对比，岩石学、元素地球化学和硅、氧同位素地球化学特征揭示了硅质岩为热水成因，形成的温度可能达到100℃。这种克拉通规模的热水硅质岩需要巨大的硅质来源，而同时期华北克拉通北缘的岩浆-火山活动可以提供充足的热液硅。这种热水成因的硅质岩往往指示同时期强烈的构造活动，而与古气候演化的关系不大。