古新世-始新世极热事件(PETM)浅海生态系统稳态转变:以塔里木盆地库孜贡苏剖面钙质超微浮游植物为例

ECOSYSTEM REGIME SHIFT IN SHALLOW MARINE ENVIRONMENT DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM(PETM):A CASE STUDY OF CALCAREOUS NANNOPLANKTON FROM THE KUZIGONGSU SECTION IN THE TARIM BASIN

古新世-始新世极热事件(PETM)发生于56 Ma前,因其碳释放过程与当前人类CO_(2)持续排放驱动的全球变暖类似而备受世人关注。塔里木盆地保存完好、出露完整且连续的古近纪浅海地层为研究PETM期间东特提斯洋古环境变化提供了理想材料,但目前缺乏对PETM前后塔里木盆地浮游植物群落稳态转变的研究。本研究利用库孜贡苏剖面齐姆根组约48 m厚海相沉积物中保存的丰富钙质超微化石群落,研究PETM前后生态系统演变过程,揭示PETM事件前后发生了两次稳态转变。第一次稳态转变发生在沉积厚度约为19.61~24.56 m之间,与PETM事件开始时间恰好对应;第二次稳态转变发生在沉积厚度约为29.60~31.06 m之间,与PETM事件恢复期基本对应。PETM事件前Coccolithus pelagicus、Toweius pertusus和T.eminens等钙质超微化石占主导地位,当稳态开始发生改变时,T.pertusus和T.eminens相对丰度开始下降,而“负偏属种”Discoaster araneus和Discoaster acutus开始出现。P和Zn/Al等营养指标升高,表明东特提斯洋地区在PETM期间初级生产力增加,高生产力指示种Neochiastozygus junctus在PETM期间显著增加也印证了这个观点。北大西洋中部滨海平原沉积记录表明,其生态系统在PETM时期同样经历了两次稳态变化,与库孜贡苏剖面两次转变时间基本对应,表明由PETM事件导致的浅海生态系统稳态转变可能是全球而非区域性变化。

The Paleocene-Eocene Thermal Maximum(PETM)event occurred 56 million years ago has drawn significant attention due to its similarity to anthropogenic CO_(2) driven global warming.The Tarim Basin has well-preserved,fully exposed,and continuous Paleogene shallow marine strata,offering ideal materials for studying the paleoenvironmental changes of the eastern Tethys Ocean across the PETM.However,there is a lack of research on the regime shifts of plankton assemblages in the Tarim Basin across the PETM.This study selected the Kuzigongsu section(39°44'19″N,75°18'20″E)located in the northeastern part of Wuqia County,Kashgar region,Xinjiang,as the research site.Samples were collected at ca.10 cm intervals,resulting in a total of 480 samples with a total thickness of the section ca.48 m from the Qimugen Formation.By selecting one out of every four samples,123 samples were obtained for calcareous nannofossil study.PCA analysis and STARS mutation detection of the abundant calcareous nannofossil assemblages from the Kuzigongsu section revealed two significant regime shifts across the PETM.The first regime shift occurs between sediment thicknesses of approximately 19.61 m to 24.56 m and coincides with the PETM onset.The second regime shift occurs between sediment thicknesses of approximately 29.60 m to 31.06 m,roughly corresponding to the PETM recovery.Prior to the PETM,the calcareous nannofossil assemblages were dominated by Coccolithus pelagicus,Toweius pertusus,and T.eminens,with an average relative abundance of 61.6%.As the ecosystem shifted to another stable state,the relative abundance of T.pertusus and T.eminens decreased,with average values decreasing from 20.5%and 8.0%before the PETM to 13.0%and 3.3%during the PETM,respectively.At the same time,"excursion taxa"such as Discoaster araneus and Discoaster acutus emerged.Increases in nutrient index such as phosphorous(P)concentration and Zn/Al ratio suggest elevated primary productivity in the eastern Tethys Ocean during the PETM,which is consistent with the significant increase of the high-productivity Neochiastozygus junctus.Our analysis also showed that the shallow marine ecosystem in the North Atlantic,as reconstructed from the sedimentary record off Maryland shelf,also experienced two regime shifts across the PETM,which largely correspond to the timing of those in the Tarim Basin.This further suggests that the regime shifts in shallow marine ecosystems caused by the PETM may be a global rather than a regional change.