摘要:
为研究中国沙漠东部广泛分布的风成沙—古土壤沉积序列的颜色变化与古气候之间的可能联系,以浑善达克沙地为研究区,测定了7个天然风成沙—古土壤沉积序列的颜色(L*a* b*)、有机碳、总氮和粒度。结果表明:L*(亮度)、a*(红度)、b*(黄度)值分别介于32.41~65.89,2.73~7.52和-1.68~17.16;L*、a*、b*值之间存在正相关关系,其中L*和b*之间的相关性最显著(R2=0.90),而L*和a*之间的相关性最低(R2=0.26);L*、a*、b*值在垂直方向上具有相同的变化趋势和特征,它们在风成沙层位处为高值,在灰色—黑色砂质古土壤层位处为低值,但L*值变化较大,a*和b*值变化较小。均值的非参数检验表明L*可指示风成沙—古土壤沉积序列的风成沙和古土壤层的变化,而a*在这类沉积序列上变化不敏感。影响L*的主要因素与有机质含量和成壤作用有关,它们都与东亚夏季风强弱变化关系密切,因此L*有明确的古气候意义;L*低值对有机质含量增加和成壤作用的加强具有指示作用,是东亚夏季风增强的信号。基于颜色参数L*可以很好地重建中国沙漠东部风成沙—古土壤沉积序列的古气候变化。
Abstract:
The alternating units of dune sands and paleosols are widely distributed acorss arid and semi-arid region of Northern China. To study the possible relationship between sand-paleosol sequences color and paleoclimate, three color parameters, lightness (L*), redness (a*) and yellowness (b*), of seven relict sand-paleosol profiles across Otindag sandy land were measured with a colorimeter. The results show the values of L*, a*, b* are 32.41~65.89, 2.73~7.52 and -1.68~17.16, respectively. Three color parameters have significantly positive relationship, correlation between L* and b* is strongly significant (R2 = 0.90), but that of L* and a* is quite low (R2=0.26). Although lightness, redness and yellowness have similar trend curves along sand-paleosol profiles, their variations are distinctly different. The changes of L* value are significantly larger than that of both a* and b* values. Nonparametric tests of lightness, redness and yellowness on sand-paleosol profiles indicate that lightness and yellowness are significantly different between sand and paleosols layers, with relatively low lightness and yellowness values in the paleosols layers. By contrast, the redness could hardly indicate their differences between sand and paleosols layers. Variations of the lightness are strongly related to the contents of organic matter, clay and silt. Relative effects of these factors on the lightness decrease gradually from organic matter and clay to silt. These changes are founded to be due to variations in the regional patterns of precipitation, which is possibly attributed to cyclic oscillations of East Asian summer monsoon precipitation. Therefore, the lightness (L*) could be used as a proxy indices to reconstruct past summer monsoon variability. Low values of L* possibly indicate a relatively high organic matter and a strengthening signal of East Asian summer monsoon, and vice versa. These results suggest that quantitative color determination of sand-paleosol sequences using a colorimeter could provide a new clue to understand the East Asian summer monsoon changes.
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