华北南部晚石炭-早二叠世沉积记录对晚古生代冰期全球气候变化的启示

Glaciation in the late Paleozoic ice age was discrete with its acme and subsequent demise in the Late Carboniferous-Early Permian in association with the climate transition from icehouse to greenhouse. The glacial sedimentary and stratigraphic records in the high-latitude Gondwana can be correlated approximately in time with the indirect chemical indicators in the low-latitude tropical-subtropical regions represented by the cyclic sedimentary sequences and the C-O isotopes of carbonates and bio-shells, denoting climate changes in the late Paleozoic ice age. However, the uncertainty in chronostratigraphy and unequivalence in climate indicator used for climatic correlation between low and high latitudes hinder our further understanding of the global climate changes in the late Paleozoic ice age. For the Late Carboniferous-Early Permian clastic sequence in the glacial high-latitude Gondwana there have been lots of element geochemical and organic carbon isotopic data reported. While, there are few tightly age-constrained equivalent data in the low-latitude regions. North China was in the equatorial region during the Late Carboniferous-Early Permian and there developed a coal-bearing clastic sequence with volcanic tuff and marine limestone interbeds, providing a low latitudinal clastic sedimentary record to compare with the high-latitude ones. This study aims to promote the chronostratigraphic precision of this sequence based on high-precision tuff zircon dating and limestone biostratigraphy, reveal the climate significances of mudstone weathering geochemistry and organic carbon isotope composition integrated with sandstone provenance and sedimentology analysis, then make an equivalent-chemical-index based climate correlation between low and high latitude records under finer time scale, and discuss the global climate changes and climate-related factors as well.

晚古生代冰川活动在晚石炭-早二叠世达到鼎盛并随后趋于衰减，全球气候由冰室向温室转变。高纬地区冰川活动的沉积和地层记录是气候变化的直接指标，而低纬地区的旋回沉积和碳酸盐岩-生物壳C-O同位素则为气候变化提供间接指示，两者可大致对比反映全球气候变化。然而，晚古生代冰期气候变化的全球对比仍受限于高纬和低纬地区间地层时代的不确定性和气候指标的不对等性。华北板块在晚石炭-早二叠世处于低纬地区，沉积序列以碎屑岩为主，夹有多层凝灰岩和灰岩,适合与高纬碎屑岩沉积进行对比研究。本项目拟以华北南部晚石炭-早二叠世沉积记录为研究对象，通过凝灰岩高精度锆石U-Pb定年，结合灰岩生物地层学提高地层年代精度，并综合砂岩物源分析和沉积相解释，识别泥岩风化地球化学和有机碳同位素指标对古气候变化的响应，进而与高纬地区进行精细时间格架下基于相同地化指标的沉积记录对比，探讨晚古生代冰期全球气候变化及气候相关因素的内在联系。

晚古生代冰期是显生宙以来持续时间最长的冰室气候期，其在二叠纪早期达到鼎盛，随后趋于消融，最终演变为中生代的温室气候。二叠纪的气候变化代表了地球最近一次从冰室到温室气候的状态转变事件。通过海洋钙质沉积记录和气候模型模拟，前人对晚古生代冰川的形成、鼎盛和消亡及其海水古温度变化进行了大量研究，然而对这一时期低纬区陆地古气候的变化还缺少足够了解，对气候控制机制也存在较大争议。本项目对华北南部发育较为连续的晚石炭-早二叠世沉积地层进行了详细的沉积学、岩石学、生物地层学、地质年代学和地球化学研究，在地层时代和古气候演化等方面取得了如下重要成果：（1）收集了现代大陆基岩表层土壤化学风化的有关数据，分析了温度、降雨量和物理侵蚀对表层土壤化学风化强度的控制作用，基于化学风化强度τNa与陆地表层温度MAT的相关关系建立了τNa-MAT转换方程，并据此估算了华北南部早二叠世Sakmarian晚期的表层温度为~20℃；（2）进一步挖掘泥岩的源区化学风化趋势对气候变化的指示意义，提出冰川消融期低-高纬区泥岩化学风化强度差异的减小指示了纬向陆地温度梯度的降低；（3）利用化学剥蚀-热电离质谱（CA-TIMS）技术对凝灰岩进行了高精度锆石U-Pb同位素定年，确定了3个高精度地层年龄，即301.13 ± 0.20/0.21/0.39 Ma、299.32 ± 0.12/0.14/0.35 Ma和295.65 ± 0.08/0.11/0.34 Ma，据此计算了平均沉积速率，并利用内插法约束了从本溪组顶部到下石盒子组底部的精确地层时代，建立了~301-290 Ma的高精年代地层格架；（4）根据高分辨泥岩样品的化学风化系数，建立了华北南部~301-290 Ma的化学风化趋势，并依据τNa-MAT转换方程重建了相应陆地表层古温度的变化，确定了石炭-二叠纪之交先变暖后变冷的气候变化模式及其与Skagerrak大火成岩省玄武质火山岩浆活动主期的等时关系，明确了大火成岩省驱动气候变暖和变冷的可能机制。