Application of cave monitoring to constrain the value and source of detrital 230Th/232Th in speleothem calcite: Implications for U-series geochronology of speleothems
Application of cave monitoring to constrain the value and source of detrital 230Th/232Th in speleothem calcite: Implications for U-series geochronology of speleothems
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DOI：
10.1016/j.palaeo.2022.110978
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发表时间：
2022-04
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通讯作者：
B. Wortham;J. Banner;E. W. James;R. Edwards;S. Loewy
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作者：
B. Wortham;J. Banner;E. W. James;R. Edwards;S. Loewy
洞穴方解石沉积物（“洞穴沉积物”）是广泛使用的陆地古气候信号档案，在很大程度上是因为它们可以准确和精确地使用质谱测量的238 U系列不平衡（U系列定年）。当石笋中的生长层在方解石沉积过程中含有大量的230 Th（“碎屑230 Th”）时，困难就出现了。用230 Th碎屑对U系年龄进行校正，可采用2种方法：（1）假定230 Th碎屑含量（以230 Th/232 Th比值表示）;（2）建立石笋等时线模型。我们研究了两种替代的碎屑230 Th/232 Th校正方法：分析德克萨斯州中部洞穴中人工基质上生长的方解石（即，现代或“零年龄”方解石），并分析了洞穴补给区的土壤沥滤液。在三年多的时间里收集的样品产生了更高的碎屑原子230 Th/232 Th值，范围从5.4到28.1 ppm ± 0.5 ppm（典型测量的1σ不确定性），而通常使用的“块状地球”值为4.4 ppm。协议之间的土壤渗滤液和方解石230 Th/232 Th值表明，土壤材料是在得克萨斯州中部的石笋碎屑Th的来源。铁，锰，钍的浓度之间的相关性从上述洞穴的土壤渗滤液中表明，钍吸附到铁和锰胶体物质。对得克萨斯州中部一个年轻（< 100年）石笋中碎屑230 Th/232 Th的独立估计显示，230 Th/232 Th（~4至11 ppm）高于散装土，与零龄方解石和土壤的测量结果（~5至20 ppm）一致。这些结果为完善石笋U系年表提供了一种新的方法。
Cave calcite deposits (“speleothems”) are widely-used archives of terrestrial paleoclimate signals, in large part because they can be accurately and precisely dated using238U-series disequilibria (U-series dating) measured by mass spectrometry. Difficulties arise when growth layers in stalagmites incorporate detrital material during calcite deposition that contains significant amounts of230Th (“detrital230Th”). U-series ages must be corrected for detrital230Th either by 1) assuming a detrital230Th content (expressed as the ratio230Th/232Th), or by 2) developing isochron models for each stalagmite. We examine two alternative correction approaches for detrital230Th/232Th: analysis of calcite grown on artificial substrates in central Texas caves (i.e., present-day or “zero-age” calcite) and analysis of soil leachates from the recharge zones of the caves. Samples collected over three years yield elevated detrital atomic230Th/232Th values, ranging from 5.4 to 28.1 parts per million (ppm) ± 0.5 ppm (1σ uncertainty for a typical measurement), compared with the commonly used “bulk earth” value of 4.4 ppm. Agreement between soil leachate and calcite230Th/232Th values suggests that soil material is a source of detrital Th in the stalagmites in central Texas. Correlations between Fe, Mn, and Th concentrations in soil leachates from above caves suggests that Th sorbs on to Fe and Mn colloidal material. Independent estimates of detrital230Th/232Th in a young (< 100 years) stalagmite from central Texas reveal higher230Th/232Th (~4 to 11 ppm) than bulk earth, consistent with measurements from zero-age calcite and soil (~5 to 20 ppm). These results offer a new method for improving U-series chronologies of stalagmites.