Paleoclimate Change in China: Rock Magnetic Indicators and Their Validation

点击翻译按钮获取中文摘要

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.