Molybdenum isotope fractionation in soils: Influence of redox conditions, organic matter, and atmospheric inputs
Molybdenum isotope fractionation in soils: Influence of redox conditions, organic matter, and atmospheric inputs
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土壤中钼同位素分馏：氧化还原条件、有机质和大气输入的影响
DOI：
10.1016/j.gca.2015.04.007
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发表时间：
2015
影响因子：
5
通讯作者：
Siebert C
中科院分区：
文献类型：
--
作者：
Siebert C
钼同位素分馏伴随着土壤发展的研究跨越三个成壤梯度，包括一系列的控制因素。这些因素包括可变的氧化还原条件，有机质含量，铁和锰氧（氢）氧化物含量，矿物成分，风化程度，pH值，类型和大气输入量，年龄，气候和下伏岩石类型。从毛伊岛（夏威夷）沿着850 - 5050 mm平均年降水量梯度的土壤剖面显示，土壤δ 98 Mo的平均值从最干燥、最富氧地点的−0.04 ± 0.11‰（与玄武岩母质（−0.09 ± 0.08‰）无法区分）下降到最潮湿、最还原地点的−0.33 ± 0.10‰。6种冰岛土壤在风化程度较高、有机质含量较高的土层中，δ 98 Mo值较重（达+1.50 ± 0.09‰）。从不同的土壤成分中的钼的选择性提取表明，与有机质和硅酸盐或钛氧化物残留物的协会占主导地位的保留在这些土壤中的钼，与铁和锰的氧（氢）氧化物上的吸附起较小的作用。在所有玄武质土壤中，相对于母质而言，表现出最大净Mo损失的土壤中的δ 98 Mo值较轻，而表现出净Mo增加的土壤中的δ 98 Mo值较重。在波多黎各卢基略山脉的石英闪长岩上发育的排水良好的风化层剖面显示，尽管相对于基岩，Mo的总体损失为28%，但土壤和较浅的腐泥土中的δ 98 Mo值比母质重（高达+0.71 ± 0.10 ‰，综合剖面平均值为+0.28 ± 0.10‰）。然而，更深的腐岩未从基岩中分馏出来（-0.01 ± 0.10‰，石英闪长岩基岩），表明岩石风化溶解过程和次生粘土形成不会使Mo同位素分馏。我们的数据表明，钼的质量平衡和同位素组成的土壤的氧化还原条件，有机质和大气输入控制。这样，钼同位素就有可能对风化环境中气候驱动的变化做出反应并记录下来。同位素轻和重钼（相对于母质）的存在下，在所有网站和个别土壤剖面表明，这是正常的多分馏机制下运行的开放系统条件下的土壤。
Molybdenum isotope fractionation accompanying soil development is studied across three pedogenic gradients encompassing a range of controlling factors. These factors include variable redox conditions, organic matter content, Fe and Mn oxy(hydr)oxide content, mineral composition, degree of weathering, pH, type and amount of atmospheric inputs, age, climate, and underlying rock type. Soil profiles from the island of Maui (Hawaii) along a precipitation gradient ranging from 850 to 5050 mm mean annual precipitation show a decrease in average soil δ98Mo from −0.04 ± 0.11‰ at the driest, most oxic site, which is indistinguishable from the basalt parent material (−0.09 ± 0.08‰), to −0.33 ± 0.10‰ at the wettest, most reducing site. A suite of 6 Icelandic soils display a broad trend with heavier δ98Mo values (up to +1.50 ± 0.09‰) in soil horizons that are more weathered and have higher organic matter content. Selective extractions of Mo from different soil components indicate that the association with organic matter and silicate or Ti-oxide residue dominates retention of Mo in these soils, with adsorption on Fe and Mn oxy(hydr)oxides playing a lesser role. Across all basaltic soils, δ98Mo values are lighter in soils that exhibit the most net Mo loss relative to the parent material, and δ98Mo values are heavier in soils that exhibit net Mo gains. A well-drained regolith profile in the Luquillo Mountains of Puerto Rico developed on quartz diorite shows heavier δ98Mo values than the parent material (up to +0.71 ± 0.10‰ with an integrated profile average of +0.28 ± 0.10‰) in soil and shallower saprolite, despite overall moderate loss of 28% of Mo relative to the bedrock. However, the deeper saprolite is unfractionated from bedrock (−0.01 ± 0.10‰, quartz diorite bedrock) indicating that rock weathering dissolution processes and secondary clay formation do not fractionate Mo isotopes. Our data suggest that the Mo mass balance and isotope composition of soils are controlled by redox conditions, organic matter, and atmospheric inputs. In this way Mo isotopes have the potential to react to and record climate driven changes in the weathering environment. The presence of both isotopically light and heavy Mo (relative to parent material) across all sites and within individual soil profiles suggests that it is normal for multiple fractionation mechanisms to operate under the open-system conditions of soils.