若尔盖盆地ZK3孔光释光测年及年代序列

Luminescence dating and chronological sequence of ZK3 core in Zoige Basin

关键词:  若尔盖钻孔  石英  钾长石  年代序列

摘要:

若尔盖盆地地处青藏高原东部，盆地内更新世、全新世沉积物的绝对年代数据相对匮乏。ZK3钻孔位于若尔盖盆地西北黑河和黄河之间，其岩芯为滨-浅湖、河漫、河床、堤岸亚相的浅灰色、灰绿色黏土、 粉砂、细砂，对其年代准确测定有助于认识该区域地质地貌及气候、环境演化。该文选取ZK3钻孔岩芯 106.8 m以上不同深度11个光释光样品，提取细颗粒石英、细颗粒混合矿物、粗颗粒石英和粗颗粒钾长石， 分别应用细颗粒石英简单多片再生（SMAR）、粗颗粒石英简化小测片标准曲线（SA-SGC）、钾长石高温红外后红外（pIRIR₂₂₅）和多片多步高温红外后红外（MAR-MET-pIRIR）程序测量等效剂量（D_e ）。结合光释光和AMS ¹⁴C结果，获得200 ka的年龄序列。ZK3钻孔样品光释光年龄结果总体上符合地层层序。基于ZK3 孔光释光年龄和AMS 14C年龄建立了钻孔岩芯年龄–深度框架，钻孔0—6.9 m为10 ka以来的全新世河流相沉积，6.9—46.4 m为10—100 ka晚更新世河流相-湖相沉积，46.4—106.8 m为100—200 ka中更新世湖相沉积。

Abstract:

Background, aim, and scope The Zoige Basin is located on the eastern Qinghai-Xizang Plateau, where absolute chronological data for middle-late Pleistocene to Holocene sediments remain scarce. Establishing an accurate chronology for this region is therefore crucial for understanding the evolution of its landforms, climate, and environment. This study aims to reconstruct the chronological framework of the Zoige Basin based on samples and sedimentary records from the ZK3 core. Materials and methods The ZK3 core (33.9°N, 102.3°E; 3400 m above sea level) is situated between Heihe River and Yellow River in the northwestern part of Zoige Basin. Dominant lithologies include light-gray to gray-green clay, silt, and fine sand, representing shallowlacustrine, floodplain, and channel sub-facies. Eleven samples from depths above 106.8 m were collected to extract fine-grained (4—11 μm) quartz and polymineral, coarse-grained (90—180 μm) quartz, and coarse-grained (90—180 μm) K-feldspar. Equivalent doses (De) were measured by SMAR, SA-SGC, pIRIR₂₂₅ and MAR-METpIRIR protocols, respectively. Results A chronological framework spanning about 200 ka was obtained by combining ages of optical stimulated luminescence (OSL) and AMS 14C. Results show that the ages are generally consistent with stratigraphic sequence. Discussion Both quartz and K-feldspar exhibit high sensitivity and follow single-exponential dose-response curves. The D_e of the central age model (CAM) and the minimum age model (MAM) of the coarse quartz (CQ) grains above 15.4 m in ZK3 are consistent, indicating that CQ OSL signals experienced sufficient bleaching before deposition. Below 15.4 m, quartz OSL signals approach saturation, limiting reliable quartz-based dating to <60 ka. In contrast, K-feldspar from 32.6—106.8 m yield a well-defined chronological sequence, demonstrating clear advantages for dating deposits older than 60 ka and extending the datable range to about 200 ka. OSL ages from the ZK3 core are stratigraphically coherent and align with previously published ages from the nearby ZB13-C2 core. Conclusions Based on luminescence ages and AMS 14C ages, an age-depth model for the core was established: 0—6.9 m represents Holocene fluvial sediments (<10 ka); 6.9—46.4 m corresponds to Late Pleistocene fluvial-lacustrine sediments (about 10—100 ka); and 46.4—106.8 m represents Middle Pleistocene lacustrine deposits from 100 ka to 200 ka. Recommendations and perspectives The age-depth model reveals the changes of sedimentary characteristics, which are related to the changes of sedimentary dynamics. Due to the large interval of samples, reliable deposition rates cannot be obtained. In addition, whether the alternate appearance of silt, silty clay and fine sand indicates the existence of multiple sets of sedimentary cycles in the drilled area can be further explained in combination with particle size parameters in the following study.