中元古代硫循環異常環境的地球微生物學過程研究

史曉穎++張世紅++張傳恒++馮慶來++杜遠生

摘 要：華北中元古界發現大量生物成因凝塊石，超微組構與有機礦物研究表明有機礦化是微生物巖形成的普遍過程。微生物巖受微生物類群與環境條件控制，能夠提供早期海洋環境與微生物群落的特征及其相互作用信息。研究發現了中元古代太陽黑子周期性活動的地質記錄，證明黑子活動對微生物活性和沉積具有重要影響；為揭示早期日—地—生物相互作用機理提供了重要信息，拓展了地球生物學研究的方向。在燕山盆地中元古代發現了兩個重要的生物群更替事件，分別與海洋化學變化以及火山活動導致的營養鹽輸入相關。發現了多種真核生物化石，包括具刺疑源類、多細胞絲體、瓶狀化石、梭型化石以及具復雜紋飾類型。表明這個時期真核生物已經出現明顯的多樣性分化，為進一步認識早期真核生物生物進化提供了重要參考。中元古代海洋DIC庫經歷了逐漸減小的演變；海洋硫酸鹽濃度較低，氧化還原界面較淺。在淺水環境自養微生物控制了碳的代謝過程；而深水環境中，厭氧微生物代謝對海水化學組成有很大影響。氧化還原敏感元素研究表明中元代海水以Mo虧損的缺氧水體為主，但局部層段Mo等元素富集，反映海洋在1.64 Ga～1.56 Ga有重要變化，以硫化為主。在高于莊組底和上部分別獲得鋯石SHRIMP U-Pb年齡（1626±21）Ma、（1564±39）Ma，為中元古代年代地層學提供了重要約束。中元古代不同層段黑頁巖的活性鐵組分研究表明，多數貧鐵氧化物，但含黃鐵礦很少；FeHR/FeT和FeT/Al的比值平均值分別為0.28和0.57，表明多數層段形成于缺氧但非硫化環境，證明深海以廣泛的鐵化條件為主，但間夾有間歇性分布的硫化時段。華南晚埃迪卡拉紀-早寒武世的N、C同位素研究發現δ13Corg在淺水區和深水區存在顯著差異，證明海洋仍具有缺氧分層的高梯度狀態。N同位素在早寒武世早期有明顯正異常，可能與缺乏固氮作用相關；隨后趨于正常，氧化明顯并伴生多細胞生物發育。硫化水體控制營養元素以及N、C、S循環，并影響生物演化。

關鍵詞：元古代中期 微生物巖 微生物群 穩定同位素 氧化還原敏感元素 海洋化學條件 華北地臺

"Geomicrobiological Processes in the Mesoproterozoic Ocean with Anomalous Sulfur Cycle" Anaual Report

Shi Xiaoying Zhang Shihong Zhang Chuanheng Feng Qinglai Du Yuansheng

（China University of Geosciences（Beijing））

Abstract：Abundant thrombolites were found in the Mesoproterozoic of the North China Platform. Ultra-fabrics and organo-minerals in the thrombolites suggest that organo-mineralization is a key process for the formation of microbialites， which was mainly controlled by microbe groups and the environment factors. We have also identified geological records of Sun spot activities in the Mesoproterozoic， and argued that the fluctuation of Solar energy input had exerted a great influence on the microbial activity and sedimentation. Two important biotic events were recognized in the Mesoproterozoic Yanshan Basin， which were likely related to the changes in ocean chemistry and enhanced nutrient influx. Many potential eukaryote fossils were found， including acanthomorphic aritarchs， multi-cellular algae filaments， vase-like， and ornamented fossils. This suggests a noticeable eukaryote diversification. The study shows that DIC reservoir of the Mesoproterozoic had reduced through time， with low sulfate concentration and shallow chemocline. In the shallow water environments， carbon was cycled mainly by autotrophics， while in deep waters， anaerobic microbes played important role in changing seawater chemical composition. Study of redox-sensitive elements reveals that the Mesoproterozoic ocean was dominated by Mo-depleted anoxic state. Mo-enrichment was identified in the sediments of 1.64 Ga～1.56 Ga， suggesting a change in ocean chemistry to euxinic. Fe speciation from several black shale intervals in the Mesoproterozoic reveals that most of them have low ferric oxides， with rare pyrites. The average values of FeHR/FeT and FeT/Al are 0.28 and 0.57 respectively， suggesting the intervals deposited in anoxic， bur not euxinic conditions， and confirming that the deep sea was predominantly ferruginous， with intermittently interspersed euxinic waterss during the time Study of N， C， and S isotopes revealed a significant gradient of δ13Corg in the Ediacaran to Lower Cambrian succession between shallow and deep water facies of South China， likely suggesting the ocean still markedly stratified， with a high redox gradient. N isotope anomaly was recognized in the lower most Cambrian，likely reflecting the absence of N2-fixation. with Multi-cellular metazoan occurrence accompanied with environment oxidation. We think that the euxinic state had controlled nutrient concentration and the N， C， and S cycles， which in turn had impacts on the biotic evolution.

Key Words：Mesoproterozoic； Microbiolite； Microbial assemblages； Stable isotopic compositions； Redox-sensitive elements； Ocean chemistry； North China platform

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