Identification And Contribution Of Aeolian Dust In Basalt Weathered Soil

In the process of soil formation,we usually assume a top-down development model,that is,the soil is formed by weathering of the bedrock below it.This model does not consider the contribution of wind and dust to the soil,and this contribution cannot be ignored.The loess deposit area in China,which extends for thousands of kilometers,reflects the great contribution of aeolian dust to the formation of soil,but in more atypical aeolian depositional soils,research on the contribution of aeolian dust is seriously insufficient.Due to the high similarity between the mineral and chemical composition of the aeolian dust and the average composition of the upper continental crust,this makes it difficult for us to identify the aeolian dust in the general soil.There is a huge difference in mineral and chemical composition between basalt and aeolian,which makes the distinction between the two possible.This paper selected two Cenozoic basalt areas in eastern China-Wudalianchi Volcanic Fumarole in Heilongjiang province and basalt platform in Jiangsu and Anhui,which have significant differences in climate and aeolian deposition rate,and collected samples of basalt bedrock,weathered bedrock,topsoil and weathered section.The geochemical composition of the main minerals,clay minerals,major and trace elements,Sr and Nd isotopes of the sample were systematically analyzed,and the geochemical signals of aeolian dust were identified from the samples,and the contribution of aeolian dust to the weathering of basalt soil was estimated,and 300 was calculated.The wind and dust fluxes in the Wudalianchi area over the past years have discussed the characteristics of weathering and soil formation of basalts in the two places.Through the above work,this paper has obtained the following preliminary results:(1)Minerals,elements,and isotopes have all confirmed that aeolian dust is widespread in basalt weathered soil.In Wudalianchi and Jiangsu-Anhui regions,all three methods have detected obvious aeolian signals.In terms of mineral composition,the soils in Wudalianchi and Jiangsu and Anhui regions contain more typical non-basalt weathering minerals such as quartz particles and illite.In terms of element composition,the Mg/Al,Mg/Ti,Sr/Nb,Sr/Th and other elements or element ratios of soil samples in the Wudalianchi area show a mixture of aeolian dust and basalt.Due to severe weathering in the Jiangsu and Anhui regions,conventional element identification methods are difficult to distinguish between wind dust and soil,but the relatively high content of K,Si and other elements in the soil far exceeds the possible enrichment of rock weathering,and the enrichment of Ti,Fe and other elements The effect also shows the phenomenon of being diluted by wind and dust.The rare earth elements in the weathered soils of the two places showed obvious negative?Eu anomalies,which were different from the normal Eu distribution of basalts,which showed obvious wind-dust signals.The ⁸⁷Sr/⁸⁶Sr value of the air-jet butterfly soil in Wudalianchi is 0.706785-0.710272,and the?Nd is-3.8 to-7.0,which is significantly different from the ⁸⁷Sr/⁸⁶Sr value(0.705535)and?Nd value(-3)of the basalts in the area.The ⁸⁷Sr/⁸⁶Sr value is 0.703721-0.716119,and the?Nd value is 4.5 to-10,which is also different from the underlying basalt.The⁸⁷Sr/⁸⁶Sr value and?Nd value of the basalt soils of the two places show a significant negative correlation,indicating that the soils of the two places It is a mixture of basalt and wind dust.(2)Based on the Nd isotope mixing model of the two ends,the contribution ratio of the aeolian dust in different samples to the soil and the aeolian flux in the Wudalianchi area in the past300 years were calculated.The calculation results show that the contribution of aeolian dust to the soil in the Wudalianchi jet dish ranges from 21%to nearly 100%,and the contribution of aeolian dust in most of the soil exceeds 40%.The contribution ratios of different soil samples vary greatly,which may be caused by the uneven distribution of volcanic ash material at the time of formation.Aeolian dust contributes less than 10%to close to 100%of the soil in Jiangsu and Anhui.The topsoil on the top of the platform with a certain slope is relatively thin,and the contribution of aeolian dust to the soil is only 10%.However,most of the thicker soil profiles have nearly 50%contribution to eolian dust,and the contribution rate of eolian dust does not change much with depth.The contribution rate of wind-dust from the sediments in the creek and the soil in the farmland around the platform is nearly 100%.The wind-dust contribution rate calculated based on the mixed mode of Nd isotope two-terminal element and?Eu anomaly has a very good coupling,indicating the reliability of the calculation results.Benefiting from the determined age of the bedrock in Wudalianchi area,the average wind-dust flux in this area in the past 300 years is calculated to be 4.38g/yr·m².(3)The basalt in Wudalianchi air-jet butterfly soil has basically no weathering,while the weathering of Jiangsu-Anhui basalt shows complete montmorillonite,which is a significant effect of removing Ca and Mg.In the accumulation of aeolian dust,the soil Si,K The content of the basalt is increased.Affected by the relatively large elevation difference,the weathering products and wind dust of the Jiangsu-Anhui basalt are easily eroded by precipitation,resulting in a thin weathering profile,which promotes the basalt weathering in a"supply limited"mode.Our research provides new evidence for further understanding of the role of aeolian dust in the geochemical cycle of elements and the weathering mechanism of basalt.