# Application of Uranium Isotope Chronology for Partical Comminution in the Eolian Dust System

Fu Yuanhe, Li Le, Chen Jun*

Ministry of Education Key Laboratory of Surficial Geochemistry, School of Earth Sciences and Engineering,Nanjing University, Nanjing 210023, China

First author:Fu Yuanhe(1993-), male, Bingzhou City, Shandong Province,Master student. Research areas include uranium isptope geochemistry. E-mail:fyuanhe@163.com

Abstract

The wind dust system is an important part of the terrestrial surface system and plays an important role in many key belts. The mechanism of wind dust and the handling process are important to understand the environmental function of wind dust and to interpret the paleoclimate record. In the past, traditional geochemical methods can only reflect the rock composition or age in the final denudation zone, and it is not possible to distinguish the different silt mechanism and the intermediate process under the same eventual source background, which is one of the biggest challenges of the present research. The 234U/238U ratio of fine matter caused by alpha decay recoil reflects the time experienced by the particle since it was broken and may be able to effectively trace the mechanism of wind dust generation and the transport of the intermediate process, but the age of uranium isotope fragmentation is rarely used in the wind dust system. The complicated factors restricting the wide application of uranium isotope were summarized, and according to the latest research progress, the verification and development of the uranium isotope comminution age in the wind dust system, and the problem solution of the mechanism of wind dust production and the way of transporting were discussed.

Keywords： Uranium isotope ; Comminution age ; Transportation process ; α-recoil ; Loess

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Fu Yuanhe, Li Le, Chen Jun. Application of Uranium Isotope Chronology for Partical Comminution in the Eolian Dust System[J]. Advances in Earth Science, 2018, 33(10): 1034-1047 https://doi.org/10.11867/j.issn.1001-8166.2018.10.1034.

## 2 铀同位素破碎年代学模型

### 2.1 原理

$λ238238$U= $λ234234$U,(1)

(a)颗粒α衰变反冲作用示意图;(b) 颗粒表面(234U/238U)与破碎年龄tcom演化图(据参考文献[40]修改)

Fig.1   Diagram showing radioactive decay of Uranium isotope
(a) Schematic diagram of recoil ejection of 234Th from a spherical grain as a result of the alpha decay of 234U, followed by beta decay of 234Th to 234U;(b)The time dependent (since the onset of weathering) evolution of (234U/238U) as a function of sediment grain size (modified after reference[40])

$234U238U$= $λ238λ234$=54.89×10-6,(2)

$234U238U=234U238Uλ234λ238 。(3)$

$234U238U=(1-fα)+234U238U0-(1-fα)e-λ234tcom,(4)$

$234U238U=1-fα(1-e-λ234tcom),(5)tcom=-1λ234ln234U238U-1fα+1,(6)$

### 2.2 搬运时间

$tT=1λ234ln1-(1-fα)234U238U-(1-fα)-tD,(7)$

## 3 铀同位素破碎年代学的难点

### 3.3 反冲系数fα的确定

fα的微小变化能引起破碎年龄的大幅度变化[40,53]。沉积物粒度变化大,表面几何形貌差别大,不能直接测量fαfα由颗粒粒度、形态和粗糙度决定,粒度与颗粒纵横比可以由传统方法测定[34],但粗糙度在不同样品中变化很大[14,20]。不同地区碎屑颗粒的fα值具有很大差异(图2)[21]

CLP:黄土高原;Hanford:华盛顿Hanford花岗质冲积物; Site 984:北大西洋984钻点钻孔; Dome C:南极冰芯Dome C;KRF:加州Kings河冲积扇; OT:冲绳海槽

Fig.2   Dependence of recoil fraction (fα ) on grain-size[21]
CLP. Chinese Loess Plateau; Hanford.Granitic fluvial sediments in Hanford,Washington; Site 984.Drill site 984 in North Atlantic; Dome C. Antarctic Dome C ice core (Site 984); KRF. Alluvial fan of Kings River,California; OT. Okinawa Trough

(1)通过体积参数计算 $fα[15]$:

$fα=∫L2rmaxX(r)β(r)λs(r)34Lr-L312r3dr,(8)$

(2)通过比表面积计算fα

$θ=K[ln(P0/P)]D-3,(9)$

$fα=142D-14-D×aLD-2LSρs,(10)$

(3)通过已达平衡的老样品的(234U/238U)计算fα:

$fα=1-234U238Uequi,(11)$

(4)通过测量226Ra和230Th计算 $fα[14,15,20]$:

$fa=3437(1-226Ra/230Th),(12)$

### 3.4 化学清洗的影响

Fig.3   Distribution of (234U/238U) in the detritus grain (modified after reference[15])

(1)区域一:(234U/238U)=1。

(2)区域二:(234U/238U)<1。

(3)区域三:(234U/238U)>1。

Fig.4   Schematic of the expected effects of leaching treatments on the 234U/238U activity ratio of sediment samples (modified after reference[51])

Fig.5   Long-term reproducibility and analytical uncertain-ties of the whole procedure using SSB method based on one loess sample of >1 Ma[21]

### 3.5 风化溶解的影响

trecoil/tdiss= $Rdiss(λ234Ldissρs)$, (13)

Depaolo等[15]假设Rdiss=2.5×10-18 (mol/(m2·s)),据此公式计算了北大西洋沉积物234U亏损必需时间与溶解时间比值,认为风化溶解对颗粒(234U/238U)的影响微弱(10%)。

### 3.6 成岩压实的影响

Depaolo等[15]认为成岩压实作用对颗粒(234U/238U)影响很小,因为234Th的反冲距离仅能使234Th的植入被局限在颗粒表面很小的部分,而且在化学清洗过程中,由234Th植入后衰变而成的234U会被酸淋滤掉。

## 4 铀同位素破碎年代学在风尘系统中的应用

### 4.1 黄土高原风尘物质的搬运时间

(234U/238U)0=1-fα×(1- $e-λ234tT$ ), (14)

$1-(234U/238U)1-(234U/238U)0$= $1-e-λ234(tT+tD)1-e-λ234tT$, (15)

4.2.1 黄土高原黄土物源

4.2.2 中国东部黄土物源

## 5 铀同位素破碎年代学在其他领域的应用

### 5.1 冰川沉积物

Aciego等[33]计算了南极洲Dome C冰芯中沉积物的破碎年龄为85~870 ka;Depaolo等[14]验证了现代冰川前缘终碛物的(234U/238U)接近长期平衡(1.00±0.01),与基岩岩性无关。

### 5.2 深海沉积物

Depaolo等[15]通过研究北大西洋沉积物,发现碎屑颗粒的搬运时间受冰期—间冰期旋回导致的沉积物源变化的影响,冰期时,沉积物搬运时间长,间冰期时,搬运时间短。李超等[38]在关于冲绳海槽沉积物的研究中,也有着与Depaolo等相同的结论。

### 5.3 河流沉积物

Dosseto等[34]通过研究澳大利亚Murrumbidgee古河道沉积物,发现在冰期—间冰期旋回过程中,流域植被类型和密度的变化导致了物质源区和搬运时间的变化。更长的搬运时间可能反映了古老沉积物的再循环[13,20,34,45]

## 6 展 望

The authors have declared that no competing interests exist.