2—3Ma沉积物石英Al心ESR测年可行性研究

Attempt of ESR dating on 2—3 Ma deposits using quartz Al center

关键词:  晚上新世  早更新世  沉积物  Al心  ESR测年

Key words:  Late Pliocene  Early Pleistocene  sediment  Al center  ESR dating

摘要:

晚新生代碎屑沉积物是研究新构造运动和晚新生代地质环境变化的重要信息载体，建立可靠的碎屑物沉积年代框架体系，是恢复构造演化和重建环境变化历史的基础和前提。目前，对＜2 Ma的沉积物断代研究，已经有较多成熟、可靠的测年方法；但对2—3 Ma不含火山灰的沉积物测年，是目前年代学研究的难点和重点，而石英 Al 心 ESR 测年法为这类样品的断代研究提供了一种可能性。以银川盆地 YNZK01钻孔721—962 m的沉积物为研究对象，探讨应用石英Al心对晚上新世− 早更新世沉积物进行 ESR测年的可行性。结果显示：（1） 在50000 Gy附加剂量辐照范围内，10个样品的石英Al心ESR信号均未显示出饱和状态；（2）“单饱和指数”函数可以很好地描述低附加辐照剂量下，Al心ESR信号强度对附加辐照剂量的响应；（3）总体来说，所获得的位于约2—3 Ma石英Al心ESR年龄和古地磁内插年龄在误差范围内一致，表明石英Al心ESR法测年为晚上新世 −早更新世沉积物断代研究提供了一种可行的绝对定年方法。

Abstract:

Background, aim, and scope The Late Cenozoic represents the most recent geological interval in which climate change, tectonic activity, and river evolution are well-preserved in sedimentary archives. Establishing precise chronological control over these processes is therefore essential for reconstructing Earth system dynamics. However, K-Ar dating of tephra, commonly used for constraining ages beyond about 200 ka, is often precluded by the absence of suitable volcanic layers in many continental sequences. Electron spin resonance (ESR) dating of quartz offers a promising alternative. Among quartz paramagnetic centers (e.g., E′ ₁, Ti-Li, Ge), the Al center is currently regarded as the most robust signal for ESR dating of Late Cenozoic sediments, particularly those of Pliocene to Early Pleistocene age. Nevertheless, its application to pre-Quaternary are remains poorly constrained. To address this gap, we applied quartz Al center ESR dating to fluvial sediments recovered from borehole YNZK01 in the Yinchuan Basin (NE Qinghai-Xizang Plateau), targeting an age range spanning the Late Miocene to Early Pliocene. Materials and methods Ten fluvial sediment samples were collected from 721 m to 962 m of borehole YNZK01 (about 1200 m deep). The lithology consists predominantly of clayey siltstone. Quartz was extracted and purified following standard protocols. ESR measurements were performed on the Al center using a Bruker EMX spectrometer. Dose-response curves were fitted using both single-saturation-exponential (SSE) and exponential-plus-linear (EXP+LIN) models. Environmental dose rates were calculated based on measured U, Th, and K concentrations and cosmic-ray contributions. Results The SSE yielded statistically superior or comparable fits to EXP+LIN for the Al center under low gamma ray doses (<12000 Gy). ESR ages range from (2.230±0.602) Ma to (3.568±0.453) Ma and agree with the independently established magnetostratigraphic framework within analytical uncertainties (age discrepancies: 11%—33%). Crucially, none of the ten samples exhibited signal saturation, even at added gamma ray doses up to 50000 Gy, confirming the suitability of the Al center for dating Late Cenozoic fluvial sediments. Discussion For the upper seven samples, ESR ages align closely with interpolated magnetostratigraphic ages within error bounds. In contrast, the three deepest samples yield ESR ages significantly older than their magnetostratigraphic interpolation results. Three non-exclusive explanations are proposed: (1) post-depositional mobility of U, Th, and K in deep fluvial-lacustrine strata may introduce substantial uncertainty into environmental dose-rate calculations; (2) existing equivalent dose (De) fitting functions may inadequately describe the radiation response of ancient, low-sensitivity sediments, thereby biasing De estimation; and (3) incomplete correction for residual signal intensity in the Al center could lead to slight overestimation of burial age. Despite these discrepancies, the bottom-three ESR ages retain stratigraphic and chronological utility as conservative upper bounds. Future work should prioritize (1) calibration using independently dated reference materials and (2) theoretical and experimental refinement of De modeling, especially for pre-Quaternary sediments, to improve accuracy and reduce systematic biases. Conclusions Quartz Al center ESR dating provides a viable and increasingly reliable geochronological tool for Late Pliocene and early Pleistocene fluvial deposits where tephra or other absolute dating methods are absent. Its demonstrated robustness against signal saturation and compatibility with magnetostratigraphy underscore broad applicability across continental sedimentary archives. Recommendations and perspectives For Late Cenozoic sediments, optimal gamma dose ranges for Al center ESR dating fall below 12000 Gy to ensure high-fidelity De determination and minimize model-dependent uncertainties.