Paleoclimate Change in China: Rock Magnetic Indicators and Their Validation

在2.4 Ma古老的中国大陆黄土/古土壤序列中，大的磁化率峰(古土壤)和大磁化率谷(黄土)与气候变化的沉积学、地球化学和古生物学特征以及来自海洋的全球冰量年代记录(18O)具有很好的相关性。为了用定量的古气候参数解释FineScale X记录，提出了两种磁源模型，一种是风成的恒定粒度磁源，另一种是古土壤中的成壤磁源，因此是细粒磁源。我们比较了距黄土源等距离的一个干旱气候和一个潮湿气候地点的四个相同的古土壤和黄土层位(最后150ka)的室温岩石磁性。首先，我们证实了其他人最近的观察是正确的：古土壤层的磁性颗粒比黄土中的更细小，这表明部分是土壤成因。接下来，我们将磁性测量扩展到低温(至15K)，以获得对超细分数的半定量估计，并通过整体粒度分布测量证实了这些估计。第三，利用附加数据(穆斯堡尔吸收光谱)，我们发现黄土中的磁性矿物并不相同：黄土层中含有较多的风成磁性矿物(L微米)，较少的成壤磁铁矿(~10 nm)。总体而言，湿润的场地确实显示出较高的成土比例，对这种成对磁深度剖面的比较可以将风成输入与成壤生产分开。这是一个扩展的供应，其中来自附近两个干旱气候地点的平均磁记录将与两个湿润气候地点的平均磁记录进行比较，以区分共同的风沙记录和区域成壤记录。磁性矿物组成和粒度的定量估计将从低温磁性和穆斯堡尔数据中获得，然后将使用成壤作用的独立测量来检验所选样品在成壤作用中的推断变化(解释为区域古季风强度变化)。总体粒度分布、(Al_2O_3+Fe_2O_3)(Na_2O+K_2O)、FeO/FeO_3、CaCO_3含量等化学指标、自生粘土矿物学、成土成熟度的扫描电子显微镜结构证据和CBD提取的可溶细粒氧化铁含量。这一结果将验证用于测量古环境变化的地磁参数，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.