Estimating the silica content and loss-on-ignition in the North American Soil Geochemical Landscapes datasets: a recursive inversion approach

Abstract: A novel method of estimating the silica (SiO2) and loss-on-ignition (LOI) concentrations for the North American Soil Geochemical Landscapes (NASGL) project datasets is proposed. Combining the precision of the geochemical determinations with the completeness of the mineralogical NASGL data, we suggest a ‘reverse normative’ or inversion approach to calculate first the minimum SiO2, water (H2O) and carbon dioxide (CO2) concentrations in weight percent (wt%) in these samples. These can be used in a first step to compute minimum and maximum estimates for SiO2. In a recursive step, a ‘consensus’ SiO2 is then established as the average between the two aforementioned estimates, trimmed as necessary to yield a total composition (major oxides converted from reported Al, Ca, Fe, K, Mg, Mn, Na, P, S, and Ti elemental concentrations + ‘consensus’ SiO2 + reported trace element concentrations converted to wt% + ‘normative’ H2O + ‘normative’ CO2) of no more than 100 wt%. Any remaining compositional gap between 100 wt% and this sum is considered ‘other’ LOI and likely includes H2O and CO2 from the reported ‘amorphous’ phase (of unknown geochemical or mineralogical composition) as well as other volatile components present in soil. We validate the technique against a separate dataset from Australia where geochemical (including all major oxides) and mineralogical data exist on the same samples. The correlation between predicted and observed SiO2 is linear, strong (R2 = 0.91) and homoscedastic. We also compare the estimated NASGL SiO2 concentrations with another publicly available continental-scale survey over the conterminous USA, the ‘Shacklette and Boerngen’ dataset. This comparison shows the new data to be a reasonable representation of SiO2 values measured on the ground over the same study area. We recommend the approach of combining geochemical and mineralogical information to estimate missing SiO2 and LOI by the recursive inversion approach in datasets elsewhere, with the caveat to validate results. Datasets: The original geochemical and mineralogical data for soils of the conterminous United States (A and C horizon datasets) were downloaded from https://mrdata.usgs.gov/ds-801/. The ‘Shacklette and Boerngen’ dataset was downloaded from https://mrdata.usgs.gov/ussoils/. A worked example for the five selected samples of Figure 5 is available as a Microsoft Excel spreadsheet (NALG_Ch_oxides_with_estimated_SiO2_LOI_worked example.xlsx) on Zenodo.org. The new datasets including sample identification, coordinates, converted major oxide concentrations, and the concentration estimates for SiO2 and LOI in wt% for the A and C horizon datasets from the North American Soil Geochemical Landscapes (NASGL) project are available as comma separated value files (NALG_Ah_oxides_with_estimated_SiO2_LOI.csv and NALG_Ch_oxides_with_estimated_SiO2_LOI.csv) on Zenodo.org.摘要： 提出一种面向北美土壤地球化学景观（North American Soil Geochemical Landscapes, NASGL）项目数据集的二氧化硅（SiO₂）与烧失量（loss-on-ignition, LOI）浓度估算新方法。本研究将地球化学测定的精准性与矿物学NASGL数据的完整性相结合，提出“反常规法”或反演方法，首先计算样品中二氧化硅、水（H₂O）与二氧化碳（CO₂）的重量百分比（weight percent, wt%）最低限值。以此为基础，第一步可计算得到二氧化硅的最低与最高估算值；随后通过递归步骤，以两类估算值的平均值构建“共识”二氧化硅浓度，并根据需要进行截尾调整，使总组分（由已报道的铝、钙、铁、钾、镁、锰、钠、磷、硫、钛元素浓度转换得到的主量氧化物 + “共识”二氧化硅 + 转换为重量百分比的已报道微量元素浓度 + “标准”水 + “标准”二氧化碳）不超过100 wt%。总组分与100 wt%之间的剩余组分间隙被视为“其他”烧失量，其主要包含已报道的非晶质相（地球化学或矿物学组成未知）中的水与二氧化碳，以及土壤中存在的其他挥发性组分。本研究利用澳大利亚的独立数据集验证该方法的有效性，该数据集的同一样品同时具备地球化学（包含所有主量氧化物）与矿物学数据。预测值与实测二氧化硅浓度呈显著线性相关（决定系数R²=0.91）且符合同方差特性。此外，本研究将估算得到的NASGL项目二氧化硅浓度与另一项覆盖美国本土的大陆尺度公开调查数据集“Shacklette与Boerngen数据集”进行对比，结果表明新数据集能够合理反映同一研究区域内实地测量的二氧化硅数值。本研究推荐在其他数据集的研究中，结合地球化学与矿物学信息，通过递归反演方法估算缺失的二氧化硅与烧失量浓度，但需注意对结果进行验证。 数据集： 美国本土土壤的原始地球化学与矿物学数据（A层与C层数据集）从https://mrdata.usgs.gov/ds-801/下载获取。 “Shacklette与Boerngen数据集”从https://mrdata.usgs.gov/ussoils/下载获取。 针对图5中选取的5个样品的计算示例可在Zenodo.org上以Microsoft Excel表格（NALG_Ch_oxides_with_estimated_SiO2_LOI_worked example.xlsx）形式获取。 包含样品标识、坐标、转换后的主量氧化物浓度，以及北美土壤地球化学景观（NASGL）项目A层与C层数据集的二氧化硅与烧失量重量百分比浓度估算值的新数据集，以逗号分隔值文件（NALG_Ah_oxides_with_estimated_SiO2_LOI.csv与NALG_Ch_oxides_with_estimated_SiO2_LOI.csv）形式发布在Zenodo.org上。