Age and origin of Terra Rossa soils in the Coonawarra area of South Australia
Age and origin of Terra Rossa soils in the Coonawarra area of South Australia
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DOI：
10.1016/s0169-555x(03)00183-1
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发表时间：
2004-03
期刊：
影响因子：
3.9
通讯作者：
A. Mee;E. Bestland;Nigel A. Spooner
中科院分区：
文献类型：
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作者：
A. Mee;E. Bestland;Nigel A. Spooner
南澳大利亚库纳瓦拉地区著名的 Terra Rossa 土壤主要由当地产生的风成碎屑组成，这些碎屑可能是在过去 120-130 ka 中积累的。使用以下方法对四种土壤剖面和相关的石灰岩和半月形沉积物进行了研究：大块土壤样品的质量平衡地球化学（主要和微量元素）、定量 X 射线衍射 (XRD) 矿物学、锶同位素 (87/86) 以及粒度分析和阳离子交换容量。这些数据表明，库纳瓦拉的 Terra Rossa 土壤具有厚厚的粘土 B 层，其地球化学成分均匀，主要成分是蒙皂石和高岭石。当使用来自底层石灰岩的硅酸盐残渣作为母体进行绘制时，质量平衡计算显示了不切实际的风化情况。真实的风化场景是用来自当地月形矿床的细粒硅酸盐材料作为母体产生的。甘比尔石灰岩硅酸盐残渣的锶同位素 (0.78) 与粘土 B 层位线 (0.726) 形成强烈对比。当地卫星的硅酸盐材料的锶同位素比 (0.725) 与 B 地平线土壤值相似。区域地质单元的锶同位素比表明，月牙层和土壤 B 层中的锶特征主要是来自古生代 Kanmantoo 页岩的风化产物，这些页岩广泛暴露在袋鼠岛和弗勒里厄半岛西侧的逆风处。对库纳瓦拉土壤剖面中三个样本（一个来自 A 地平线，两个来自 B 地平线）中的 61 个石英颗粒（单份）进行的光学（光激发光，OSL）测年表明，大多数石英砂颗粒被埋藏的时间只有几千年。然而，许多颗粒已经埋藏了数万年，其中三种颗粒的暴露年龄在 105 至 109 ka 之间。大量的年轻暴露日期代表最近在 A 地平线中暴露的石英砂，并且已经向下移动到 B 地平线。较早的暴露日期被解释为代表在风吹淤泥和粘土堆积期间或之后不久被埋藏的谷物。我们目前关于库纳瓦拉土壤风沉积时间和条件的模型是，大部分土壤是在 120-130ka 左右相对潮湿的最后间冰期期间积累的。在此期间，人们认为西部低洼地区的普拉亚-鲁内特系统特别活跃，并产生了显着的局部尘埃通量。
The famous Terra Rossa soil in the Coonawarra area, South Australia, is dominated by locally derived aeolian detritus, which probably accumulated over the last 120–130 ka. Four soil profiles and associated limestone and lunette deposits were investigated using the following methods: mass balance geochemistry of bulk soil samples (major and trace elements), quantitative X-ray diffraction (XRD) mineralogy, strontium isotopes (87/86), as well as grain-size analysis and cation exchange capacity. These data show that the Terra Rossa soil from the Coonawarra has a thick, clayey B-horizon which is geochemically homogeneous and dominated by smectite and kaolinite. Mass-balance calculations show unrealistic weathering scenarios when plotted using silicate residuum from the underlying limestone as parent. Realistic weathering scenarios are produced with fine-grained silicate material from local lunette deposits as parent. Strontium isotopes of silicate residuum from Gambier Limestone (0.78) contrast strongly with the clayey B-horizon (0.726). Strontium isotope ratios of silicate material from a local lunette (0.725) are similar to the B-horizon soil values. Strontium isotope ratios from regional geological units indicate that the strontium signature in the lunette and soil B-horizon is dominated by weathering products from the Palaeozoic Kanmantoo shales, extensively exposed upwind to the west on Kangaroo Island and the Fleurieu Peninsula. Optical (optically stimulated luminescence, OSL) dating of 61 individual quartz grains (single aliquot) from three samples in the Coonawarra soil profile (one from the A-horizon and two from the B-horizon) shows that most of the quartz sand grains have been buried for only a few thousand years. Many of the grains, however, have been buried for tens of thousands of years with three grains having exposure ages of between 105 and 109 ka. The large population of young exposure dates represents quartz sands recently exposed in the A-horizon and which have been translocated down to the B-horizon. The older exposure dates are interpreted as representing grains that were buried during or soon after the accumulation of wind-blown silt and clay. Our current model concerning the timing and conditions of aeolian deposition of the Coonawarra soil is that much of it accumulated during the relatively wet, last interglacial period around 120–130 ka. During that time span, it is thought that the playa–lunette systems in the low-lying areas to the west were particularly active and generated a significant local dust flux.