Paleoclimate Change in China: Rock Magnetic Indicators and Their Validation

在2.4 Ma古中国大陆黄土/古土壤序列中，大磁化率峰（古土壤）和大磁化率谷（黄土）与沉积学、地球化学和古生物学的气候变化特征以及海洋定年的全球冰量记录（18O）有很好的相关性。为了从定量古气候参数的角度解释细尺度X记录，提出了两种模式，其中磁源要么是风成的和恒定粒度的，要么是成土的，因此是古土壤中的细粒度。我们比较了四个相同的古土壤和黄土层序（历时150ka）的室温岩石磁性，这些层序来自一个干旱气候和一个湿润气候地点，与黄土源区距离相等。首先，我们证实了其他人最近的观察是正确的：古土壤层中的磁性粒度比黄土中的更细，表明部分成土成因。接下来，我们将磁性测量扩展到低温（至15K），以获得超细分数的半定量估计，并通过体粒度分布测量来证实它们。第三，利用附加数据（穆斯堡尔吸收光谱）表明，黄土层位的磁性矿物不相同：风成磁性矿物（l μ m）较多，成土磁铁矿（~10 nm）较少。总体而言，潮湿地区确实显示出较高的成土比例，并且这种配对磁深度剖面的比较可以将风成输入与成土生产分开。这是一种扩展供应，将两个附近干旱气候点的平均磁记录与两个潮湿气候点的平均磁记录进行比较，以将共同的风成记录与区域成土记录分开。将从低温磁和穆斯堡尔数据中获得磁性矿物组成和粒度的定量估计，然后使用独立的成土变化测量方法对选定样品进行推断的成土作用变化（解释为区域古季风强度变化）进行检查。体积粒度分布、（A1 2O3 + Fe2O3） （Na2O +K22O）、FeO/FeO3、CaCo3含量等化学指标、自生粘土矿物学、成土成熟的SEM结构证据和CBD萃取可溶性细粒氧化铁含量。研究结果将验证用于测量古环境变化的磁参数，以及全球（即亚洲大陆）和中国中部区域气候变化的历史。

In the 2.4 Ma old continental Chinese loess/paleosol sequences large magnetic susceptibility peaks (in paleosols) and troughs (in loess) correlate very well with sedimentological, geochemical and paleotological signatures of climate change, as well as with the dated global ice volume records ( 18O) from the oceans. For interpreting the finescale X record in terms of quantitative paleoclimatic parameters two models have been proposed in which the magnetic source is either eolian and of constant grainsize, or pedogenic and thus finer-grained in the paleosol. We have compared the room temperature rock magnetic properties of a sequence of four identical paleosol and loess horizons (that last 150ka) from one arid climate and one humid climate site, equidistant from the source of loess. First, we confirm that recent observations by others are correct: that the magnetic grainsize is finer in the paleosol layers than in the loess, suggesting a partly pedogenic origin. Next, we have extended the magnetic measurements to low temperatures (to 15K) to obtain semi-quantitative estimates of the ultrafine fraction and confirmed them by bulk grainsize distribution measurements. Thirdly, using additional data (Mossbauer absorption spectra), we have shown that the magnetic mineral is not the same: there is more eolian magnetic ( l um) and less pedogenic maghemite (~10 nm) in the loess horizons. Overall, the humid site does show a higher pedogenic fraction and a comparison of such paired magnetic depth profiles can separate the eolian input from the pedogenic production. This is an extended supply in which the average magnetic record from two nearby arid climate sites will be compared with the average of two humid climate sites, to separate the common eolian record from the regional pedogenic one. Quantitative estimates of magnetic mineral composition and grainsize will be obtained from combined low temperature magnetic and Mossbauer data, and then the inferred variations in pedogenesis (interpreted as regional paleomonsoon strength variations) will be checked for selected samples using independent measures of pedogenic alterations. bulk grainsize distribution, chemical indices such as (A1 2O3 + Fe2O3) (Na2O +K22O), FeO/FeO3, CaCo3 content etc., authigenic clay mineralogy and SEM textural evidence of pedogenic maturation and CBD- extracted soluble finegrained iron oxide content. The results will be a validation of the magnetic parameters used to measure paleoenvironmental changes, as well as the history of global (i.e., continental Asian) and regional climate changes in central China.