湖泊碎屑沉积物Mg同位素组成及对大陆风化的响应:以青藏高原达则错为例

The response of lake sediment-Mg isotope to Earth continental weathering: a case study of Dagze Co in the Qinghai-Xizang Plateau

关键词:  青藏高原  湖泊沉积物  Mg同位素  化学风化  气候变化

Key words:  Qinghai-Xizang Plateau  lake sediments  magnesium isotopes  chemical weathering  climate change

摘要:

为查明湖泊沉积物镁同位素 （δ²⁶Mg） 示踪风化强度的潜力，以青藏高原达则错湖泊30.3 cm短岩芯为研究对象，进行210Pb-137Cs定年，分析硅酸盐组分的矿物、元素和δ²⁶Mg。结果表明δ²⁶Mg变化可分为两个阶段。阶段Ⅰ：AD 1855—1930，δ²⁶Mg波动较大，从−0.16‰负偏至−1.15‰，再正偏至−0.30‰；阶段Ⅱ：AD 1930—1985，δ²⁶Mg波动较小，在−0.60‰附近波动。由于湖泊沉积物来源单一，δ²⁶Mg变化受物源影响较小；通过对比硅酸盐组分Zr/Rb比值 （反映颗粒分选）、Rb/Sr比值和伊利石/绿泥石比值 （反映风化强度），发现δ²⁶Mg变化受分选影响较小，主要受控于化学风化强度变化。对比该地区的古温度及古降水，发现二者与δ²⁶Mg变化一致，反映了区域气候对湖泊硅酸盐组分化学风化的控制作用。该研究表明湖泊碎屑沉积物镁同位素具有示踪流域风化强度及气候演变的潜力。

Abstract:

Background, aim, and scope Silicate weathering represents a fundamental sink of atmospheric CO₂ and contributes to the long-term stability of Earth’s climate. However, the response of this process to the rapid increase in atmospheric CO₂ since the Industrial Revolution remains poorly constrained. Therefore, this study aims to clarify the relationship between silicate weathering and recent climate change. To achieve this, we analyzed lake sediments from the Qinghai-Xizang Plateau, using Mg isotopes as a tracer of chemical weathering intensity in this high-elevation, minimally disturbed, and climate-sensitive archive. Materials and methods This study focuses on a 30.3 cm short sediment core from Dagze Co on Qinghai-Xizang Plateau. The core was dated using the 210Pb-137Cs method and analyzed for its silicate mineral components, elemental geochemistry, and Mg isotope compositions. Results The δ²⁶Mg values vary in two distinct stages: StageⅠ(AD 1855—1930) exhibits significant fluctuations, decreasing from −0.16‰ to −1.15‰ and then increasing to −0.30‰; Stage Ⅱ (AD 1930—1985) shows relatively stable fluctuations, varying around −0.60‰. Similarly, the illite/chlorite and Rb/Sr ratios also display a two-stage variation. In StageⅠ (AD 1855—1930): the illite/chlorite ratio fluctuates markedly between 0 and 11, while the Rb/Sr ratio shows a trend of initially increase and then decrease, ranging from 6 and 10. In StageⅡ (AD 1930—1985): the illite/chlorite ratio between 5 and 8 with reduced amplitude, and the Rb/Sr ratio fluctuates narrowly around 8 before decreasing to 6.7. Discussion The controls on Mg isotopic variations and their response to regional climate are discussed. First, as Dagze Co is semi-closed and receives detrital material mainly from the Bocangzangbu River, the sediment source can be considered stable. The weak correlation (R² =0.0115) between Zr/Rb and δ²⁶Mg suggests limited influence of grain-size sorting on δ²⁶Mg. Instead, the δ²⁶Mg variation in the silicate fraction reflects chemical weathering intensity, which is further supported by the weathering proxies (illite/chlorite ratio and Rb/Sr ratios). Second, based on Mg isotopic variations, weathering intensity, can be divided into two stages: StageⅠ (AD 1855—1930) and StageⅡ (AD 1930—1985). These stages correspond to regional variations in paleotemperature and paleo-precipitation variation, highlighting the dominant role of climate in controlling silicate weathering intensity. Conclusions These results indicate that Mg isotopes in lacustrine clastic sediments have the potential to trace weathering intensity and climatic evolution in the catchment. Recommendations and perspectives Based on our study of Dagze Co, we suggest that continental weathering is likely to intensify in the future under rising global CO₂ levels and ongoing warming.