华南二叠系卡匹敦阶高分辨率浮点年代标尺

基于详细的生物地层学研究,以磁化率为古气候替代指标,对广西来宾铁桥剖面卡匹敦阶地层开展时间序列分析,建立高分辨率浮点年代标尺(FPTS).结果表明,磁化率记录了铁桥剖面中二叠世晚期沉积序列中的米兰科维奇旋回,卡匹敦阶上部磁化率突然增加与峨眉山玄武岩喷发和卡匹敦晚期全球性海退有关,这些事件导致同期沉积物中碎屑物质增加.铁桥剖面瓜德鲁普—乐平统界线附近磁化率和蓬莱滩剖面(乐平统底界GSSP)表现出一致的变化趋势,具可对比性.利用多窗谱法(MTM)和傅里叶变换(FT)从磁化率序列中识别出五个米兰科维奇周期:长偏心率周期(E2,405ka)、短偏心率周期(E1,100ka)、长地轴斜率周期(O2,44.1ka)、长岁差周期(P2,20.95ka)和短岁差周期(P1,17.7ka).对比基于E2周期建立的磁性地层磁化率(MSS)带和标准参考带(SRZ),建立整个沉积序列的高分辨率(200ka)FPTS,提出卡匹敦阶的时限为3.85 Ma(存在+0~0.28 Ma误差),整段沉积序列的平均沉积速率为2.91cm·ka-1.同时计算出卡匹敦阶内部七个牙形石带的时限,从最短26.6ka到最长2.3 Ma.另外,估算出峨眉山大火成岩省喷发启动时间为262.67 Ma,位于瓜德鲁普—乐平统界线之下1.42 Ma.

High-resolution floating point time scale (FPTS) of Permian Capitanian Stage in South China

The Capitanian stage is a critical time interval when fierce eruption of the Emeishan large igneous province (LIP) occurred with onset of a double-episodic Paleozoic-Mesozoic bio-crisis. However, the time duration of the Capitanian stage is geochronologically poorly-constrained due to a lack of absolute age data with radioactive dating method constraining both the basal and the top boundary of the Capitanian stage. Cyclostratigraphy can provide a high-resolution astronomical time scale by tuning the cyclic stratigraphic records to astronomical solutions. In this study, a floating point time scale (FPTS) for the Capitanian stage was developed based on the result of time-series analysis of magnetic susceptibility (MS) and additional biostratigraphic data sets. In addition, we estimate the age and time duration of the Emeishan LIP eruption. 1456 unoriented samples (sampling interval ~0.1 m) for MS measurement and 45 samples for biostratigraphic study were collected from outcrops with stratigraphic interval of ~147.5 m. The bottom horizon of sampling range is located at ~31.2 m below the Wordian-Capitanian (W-C) boundary. We conducted continuous sampling with the top horizon at 2.6 m above the Capitanian-Wuchiapingian (C-W) boundary at Tieqiao section, Laibin in South China, as a supplementary reference section for the Guadalupian-Lopingian (Middle-Upper Permian) Global Boundary Stratotype Section and Point (GSSP). The stratigraphic succession consists of alternations between thin-bedded chert and micritic limestone recognized as typical slope-basin facies with carbonate cap on shallow-water platform. Representative samples were selected to be carried out thermomagnetic susceptibility measurements after analysis of low-field bulk susceptibility for all samples. Then spectral (time-series) analysis of the MS data was performed by means of Multi Taper Method (MTM) and Fourier Transform (FT) analysis. Samples for biostratigraphic study were processed using acetic acid digestion in order to obtain conodont fossils. Seven conodont zones are identified within the Capitanian stage, including Jinogondolella postserrata, J. shannoni, J. altudaensis, J. prexuanhanensis, J. xuanhanensis, J. granti, and Clarkina postbitteri hongshuiensis from the base to the top horizons of the Capitanian stage. The MS data sets are mostly negative (diamagnetic), suggesting that the Laibin area was isolated from terrigenous source during most period of the Capitanian stage. An abrupt increase in MS signal immediately below the Guadalupian-Lopingian (G-L) boundary is recorded. Although regression at the end of Capitanian might account for the increase, the coincidence of the increase with the onset of the Emeishan Large Igneous Province (LIP) eruption is intensively concerned for volcanic role in controlling the MS at the section. The results of MS and thermomagnetic susceptibility measurements indicate that the dominant magnetic constituents in this study are diamagnetic calcite and chert with subordinate paramagnetic and ferrimagnetic minerals producing the cyclicity observed in the data set. The variation of MS near the G-L boundary at Tieqiao section is essentially as same as previously published results for MS at Penglaitan Section (G-L boundary GSSP). Six Milankovitch bands are reported with eccentricity peaks E2 (~405 ka) at 0.08 cycles·m^-1 and E1 (~100 ka) at 0.35 cycles·m^-1, obliquity peaks, O2 (~44.1 ka) at 0.78 cycles·m^-1 and O1 (~35.0 ka), and precessional peaks P2 (~20.95 ka) at 1.69 cycles·m^-1, and P1 (~17.7 ka) at 1.94 cycles·m^-1. Four peaks exceed 99% confidence levels. In terms of the results of magnetostratigraphic susceptibility (MSS) zones developed from smoothed data to conform to the 405 ka (E2) eccentricity band identified in the time-series data set, a high-resolution FPTS was developed for the entire Tieqiao succession reported here. This FPTS allows a temporal resolution of ~200 ka for the studying section because each MSS zone represents half of the E2 eccentricity cycle. The results indicate that the time duration of the Capitanian Stage is estimated as ~3.85 Ma (with +0~0.28 Ma error). The mean sediment accumulation rate (SAR) for the succession is ~2.91 cm·ka^-1. Moreover, conodont zones within the Capitanian stage is estimated to spanning the shortest time interval of ~26.6 ka and the longest duration of ~2.3 Ma. In addition, the onset of the Emeishan LIP eruptions is estimated at ~262.67 Ma, 1.42 Ma earlier than the Guadalupian-Lopingian boundary.