东亚风尘钾长石矿物地球化学分布特征及其对早更新世东亚降雨演化过程的研究

Dust minerals have played a crucial role in the Earth surface system. Recent laboratory experiments have further suggested that K-feldspar act as ice-nucleating particles (Panhurst et al., 2017) that help link lithosphere and atmosphere. K-feldspar is orders of magnitude more efficient than other dust minerals in nucleating cloud-ice that has significant impact for regional climate and precipitation. These regional climatic impacts are sensitive to the concentration and distribution of K-feldspar (Atkinson et al., 2013). Its spatio-temporal distribution can provide a better understanding of East Asian precipitation, from deep time to present. The early Pleistocene has a close analogue to the future warming. However, the climatic state in the early Pleistocene remains controversial. Previous studies have suggested that the warmer early Pleistocene was drier than present (Meng et al., 2018). Rather, higher temperature was recorded from the western equatorial Pacific and may result in larger moisture transport towards East Asia. This inconsistency in the estimated climatic state of the regionally atmospheric dry versus rainy condition may largely result from the un-availability in the mineral ice-nucleate data record. In order to inform our view of the role of K-feldspar and its climatic impact to regional precipitation through the Pleistocene geologic time, this project aims to detailed mineralogy studies of the loess deposits, including changes in K-feldspar concentration. The spatially resource for high concentration in K-feldspar would be identified, together with a more detailed K-feldspar concentration in a timely manner, may enable a further constrain on the evolution of precipitation in the early Pleistocene. A better understanding of K-feldspar concentration across a range of spatial and temporal scales will improve our knowledge of precipitation state and its interaction with regional dusty climate.

风尘矿物在地球表层系统中发挥了重要作用。最近，钾长石矿物被认为具有最强的降雨凝结核效应。恢复钾长石在东亚地区的空间分布，可以帮助理解东亚降雨过程。通过研究地层中古风尘的钾长石含量变化时间序列，可能加深对过去地质历史时期东亚降雨过程与演化的理解。早更新世时期，东亚区域降雨特点存在争议。过去黄土高原的铁矿物与碳酸盐矿物研究揭示了“干夏季风”的特点，与古海洋研究获得的“强降雨”特征不符合。更为重要的是，早更新世古代风尘矿物的研究工作开展较为薄弱。本项目将通过地表砂钾长石空间分布研究,识别东亚地区高钾长石含量的区域，并通过黄土风尘钾长石矿物定量，限定早更新世风尘中钾长石矿物的演化，建立东亚区域地表对风尘中钾长石矿物的加入与早更新世东亚降雨过程及演化的可能关联。

本项目围绕“东亚风尘钾长石矿物地球化学分布特征及其对早更新世东亚降雨演化过程的研究”，主要采用场发射扫描电镜（FE-SEM）和岩石矿物解离度分析仪（MLA技术），分析了黄土高原自南向北三条黄土剖面（段家坡、灵台、赵家川）中的钾长石的矿物学特征，建立了钾长石、斜长石等矿物的定量分析方法，获得了各剖面长石族矿物（钾长石、钠长石、奥长石、中长石、拉长石和培长石）的含量时间变化序列；探讨了黄土单颗粒长石矿物微形貌特征及其与全球气候变化之间的联系。研究成果的主要创新点包括：1）研究发现黄土风尘中长石族矿物以钾长石和钠长石为主（占比超过95%），颗粒的解离程度很高，以单矿物颗粒形式存在，部分长石颗粒与粘土胶结物伴生形成土壤团聚体（100-200µm）。相比于赵家川剖面，在段家坡和灵台剖面长石颗粒显示出较强的的溶解特征。2）三条剖面的钾长石含量平均值变化不大：段家坡（8.5%, n=20），赵家川（9.3%, n=27），灵台（8.4%, n=73）。其含量随地层由老到新（3-1Ma）呈现出逐渐升高的趋势，是对早更新世全球变冷以及冰量增长的响应。3）黄土源区地表砂的钾长石含量平均值10.2%（n=38）。位于西伯利亚-蒙古板块起沙地的钾长石含量（15.7%，n=16）显著高于黄土源区的其余部分（6.1%，n=22），支持了黄土钾长石的远源假说，早更新世黄土钾长石含量的增长趋势很可能与西伯利亚-蒙古板块起沙地风速加大有关，表明这一时期全球冰量增长对冬季风增强的促进。4）黄土高原南部钾长石的颗粒平均粒径连续降低，导致钾长石颗粒集中在5-10µm，且小颗粒钾长石（5-10µm）的比例持续增加（3-0Ma），与磁化率滑动平均曲线的增长趋势一致，表明钾长石颗粒的溶解加强。早更新世夏季风降雨量的增加伴随着降温过程以及西伯利亚-蒙古板块近地表风的增强，揭示了早更新世冬、夏季风同步的古气候特点。