热带海南岛地表和地下化学风化：对全球碳循环及海水Sr 同位素演化研究的启示

热带地区火山岛/岛弧的化学风化对全球碳循环和海水的Sr同位素演化均有着重要的作用。文章对中国热带海南岛的玄武岩、花岗岩小流域和大河流域进行了河水、地下水、雨水以及基岩和沉积物的综合研究,分析了水样和固体样品的元素含量和Sr同位素比值。结果显示,河床沉积物的化学蚀变指数（CIA）与⁸⁷Sr/⁸⁶Sr之间存在着一定的变化关系,而这主要归因于沉积物的阶段性差异风化：风化早期阶段以黑云母占主导,⁸⁷Sr/⁸⁶Sr较高。此后斜长石开始风化,⁸⁷Sr/⁸⁶Sr逐渐下降。在风化程度中等阶段,斜长石大量分解后,各种富钾矿物风化加剧,⁸⁷Sr/⁸⁶Sr值升至最高点。在高风化程度阶段,随着富钾矿物逐渐减少,稳定的风化残余物质占据主导,⁸⁷Sr/⁸⁶Sr值逐渐下降。与年轻的活火山岛——加勒比海小安的列斯群岛和印度洋的留尼汪岛相比（其地下化学风化速率2~5倍于地表风化速率）,由于相对较低的降水量和老死火山低的孔隙度,海南岛地下水的流量和固溶物总量（TDS）要低的多,导致海南岛的地下化学风化速率低于地表风化速率,仅与高纬的俄罗斯堪察加活火山岛相近,为小安的列斯群岛和留尼汪岛的约6%~25%,属于全球地下风化贡献最低的区域之一。海南岛玄武岩区的地表化学风化和CO2消耗速率高于法国中央高原和西伯利亚,略低于夏威夷和德干,而显著小于东南亚的爪哇岛和 吕宋岛。在温度相近的条件下,径流量对化学风化速率有着非常明显的控制作用。由于较低的年径流量,热带区域的海南岛,其对大气CO₂的消耗能力只是处于一个全球平均的范围内。     

利用泥质岩化学蚀变指数分析物源区风化程度时的限制因素*

近年来,根据泥质岩中常量元素的摩尔数计算出的化学蚀变指数(CIA: Chemical index of alteration)、化学风化指数(CIW: Chemical index of weathering)和斜长石蚀变指数(PIA: Plagioclase index of alteration)被广泛用来反映物源区的风化程度及物源区古气候,这些指数在应用时都有一些严格的限制因素,应给予足够重视。CIA计算公式未排除成岩作用过程中钾交代作用的影响,需要采用A-CN-K[Al₂O₃-(CaO^*+Na₂O)-K₂O]三角图进行判断,对发生钾交代作用的样品利用A-CN-K三角图或CIA_corr计算公式进行校正。CIW计算公式中去掉了K₂O,但没有排除钾长石中的Al元素。PIA计算公式中考虑了钾长石中的Al元素,但只适用于判断母岩中仅含有斜长石而不含钾长石的物源区风化程度。综合分析表明,在判断物源区风化程度及古气候时,CIA的干扰因素相对较少,值得推广。但即使利用从泥质岩常量元素获得的CIA值来判断物源区的风化程度时,仍需要考虑沉积分异作用、再旋回作用、沉积区进一步风化作用以及成土作用、成岩期的钾交代作用的影响,建议首先依据泥质岩常量元素的摩尔数计算出成分变异指数(ICV: Index of compositional variability),然后对ICV>1的样品进行CIA的计算,并利用A-CN-K三角图或CIA_corr计算公式对CIA值进行钾交代作用的校正,该校正过的CIA_corr计算值可用来判断物源区的风化程度。     

峨眉山玄武岩化学风化研究——以龙苍沟小流域为例

通过对四川省雅安龙苍沟峨眉山玄武岩小流域的水化学组成研究,分析了不同物质来源对小流域溪水溶解质的贡献,并对该小流域岩石风化速率和CO2消耗速率进行了估算。结果表明,龙苍沟流域溪水呈中性,PH平均值为6.82。溪水中阳离子以Ca^2+为主,约占阳离子总量的56%;阴离子以HCO3^-为主,约占阴离子总量的45%。碳酸盐岩风化、硅酸盐岩风化、大气降水和人为活动对溪水阳离子平均贡献率分别为50.2%、38.2%、10.5%和1.1%。流域硅酸盐岩风化速率为37.54±24.94 t/km^2/yr,硅酸盐岩风化对大气C02消耗速率为5.4±3.6 mol C/km^2/yr。本文首次对我国峨眉山玄武岩省化学风化大气CO2消耗量进行估算,得到其年消耗通量为1.35±0.89×10^11 mol C/yr,约为全球玄武岩CO2年消耗通量的3.31±2.18%。     

赤水河流域岩石化学风化及其对大气CO_2的消耗

岩石风化碳汇是全球碳汇的重要组成部分,通过对赤水河流域水体主要离子组成进行测定,分析赤水河流域河水水化学特征及其岩石风化过程对大气CO_2的消耗。结果表明:赤水河流域离子组成以Ca~(2+),Mg~(2+),HCO_3~-和SO_4~(2-)为主,河水总溶解性固体(TDS)含量均值为317.88 mg/L,高于全球流域均值(65 mg/L)。元素比值分析表明赤水河流域离子组成主要受岩石风化控制,其中碳酸盐岩风化为主导控制因素,碳酸盐岩、硅酸盐岩对河水溶质贡献率分别为70.77%和5.03%。人类活动和大气降水对流域河水溶质的贡献很小。流域岩石化学风化速率为126.716 t/(km~2·a),高于黄河、长江、乌江及世界河流均值。流域岩石化学风化对大气CO_2的消耗量为10.96×10~9mol/a,岩石风化对大气CO_2消耗速率为5.79×10~5mol/(km~2·a),与长江流域接近,高于黄河流域。     

珠江流域碳酸盐岩与硅酸盐岩风化对大气CO_2汇的效应

对珠江流域11个测站的河水1个水文年4次取样进行水化学和同位素测试分析,揭示无论是碳酸盐岩区还是硅酸盐岩区,岩石风化均使河流的离子成分以HCO3-、Ca2+、Mg2+为主,碳酸盐岩风化溶蚀速率和由碳酸盐岩风化溶蚀引起的大气CO2消耗量分别为27.60 mm/ka和540.21x103mol/(km2·a-1),是硅酸盐岩风化速率和由硅酸盐岩风化引起的大气CO2消耗量的10.8倍和6.7倍,说明碳酸盐岩风化是流域碳汇过程及效应的主体。由于有利的水热条件和高的碳酸盐岩面积比例,珠江流域平均岩石风化速率和由岩石风化作用引起的大气CO2消耗量分别为30.15mm/ka和620.36×103mol/(km2·a-1),为全球60条河流平均值的2.6倍。     

硫酸参与的长江流域岩石化学风化速率与大气CO2消耗

流域的岩石化学风化过程是全球碳循环中的重要环节。以往的流域水化学碳汇通量估算大多是基于碳酸的风化作用。而实际上,硫酸和碳酸一样,也参与了流域碳元素的地球化学循环,从而对全球碳循环过程产生影响。长江流域水体近几年出现酸化现象,大部分河段SO42-和Ca2+含量增高,其对应的岩石风化过程和大气CO2消耗速率也发生变化。文章对长江干流及主要支流2013年不同季节的离子组成进行监测,利用水化学平衡法和Galy估算模型,对长江流域岩石化学风化速率和CO2消耗通量进行了估算,对硫酸参与下的长江流域岩石风化和碳循环过程进行了分析。结果表明,长江流域水体离子主要来源于硅酸盐岩风化和碳酸盐岩风化。其中碳酸盐岩风化对河水离子贡献率为92%。在硅酸盐岩广泛分布的赣江流域,碳酸盐岩风化离子贡献也达85%。分析表明,硫酸参与了长江流域的岩石风化过程,对水体中离子产生一定影响。硫酸的参与加快了碳酸盐岩的化学风化速率,平均提高约30%,但是使流域大气CO2消耗速率降低。在不考虑蒸发岩溶蚀作用下,平均从516×103 mol/km2·a降至356×103 mol/km2·a,降低约31%。在各支流中,硫酸对乌江流域碳酸盐岩的风化和碳循环的影响最大,而对雅砻江的影响最小,这与乌江流域的含煤地层、矿床硫化物及大气酸沉降有关。     

花岗岩风化程度的化学指标及微观特征对比——以香港九龙地区为例

据香港九龙3个不同工程场地取得的149件不同级别的风化花岗岩样品的化学分析结果,用统计学方法,从反映岩石风化程度的18项化学指标中选取9项进行了分析计算.对其结果及其与风化程度分级的关系的研究表明有4种指标比较理想.通过对样品微观特征的观察研究和矿物成分、微孔隙率以及综合显微岩石指标(I_p)进行了描述和定量分析,发现其中一些指标有一定相关性.这些化学指标和微观特征相关的统计值在一定程度上可定量表征花岗岩的风化程度,即:随风化作用进行,碱、碱土金属组分逐渐淋失,脱硅、富铝铁化作用逐渐加强;相应地,先是钠长石风化成高岭石,其次为云母和钾长石的风化.粘土矿物所占体积百分比随风化程度增高而变大,至残积土(Ⅵ级)时已达50%左右;I_p指标值下降,微孔隙率增高.文章综合出各个风化级别的指标变化范围及平均值供花岗岩分布区花岗岩风化野外调查和研究工作参考.     

硫酸参与的长江流域岩石化学风化与大气CO_2消耗

流域的岩石化学风化过程是全球碳循环中的重要环节。以往的流域水化学碳汇通量估算大多是基于碳酸对岩石的风化作用。而实际上,硫酸和碳酸一样,也参与了碳元素的地球化学循环,从而对全球碳循环过程产生影响。长江流域水体近几年出现酸化现象,大部分河段SO_4~(2-)和Ca~(2+)含量增高,其对应的岩石风化过程和大气CO_2消耗速率也发生变化。文章对长江干流及主要支流2013年不同季节的离子组成进行监测,利用水化学平衡法和Galy估算模型,对长江流域岩石化学风化速率和CO_2消耗通量进行了估算,对硫酸参与下的长江流域岩石风化和碳循环过程进行了分析。结果表明,长江流域水体离子主要来源于硅酸盐岩风化和碳酸盐岩风化。其中碳酸盐岩风化对河水离子贡献率为92%。在硅酸盐岩广泛分布的赣江流域,碳酸盐岩风化离子贡献也达85%。分析表明,硫酸参与了长江流域的岩石风化过程,对水体中离子产生一定影响。硫酸的参与加快了碳酸盐岩的化学风化速率,平均提高约28%。在不考虑硫酸溶蚀作用下,流域大气CO_2消耗速率平均为514.12×10~3 mol/km~2·a,但是硫酸参与时,CO_2消耗速率为467.18×10~3 mol/km~2·a,扣除碳汇量约14%。在各支流中,乌江流域受硫酸影响最大,而对雅砻江的影响最小,这与乌江流域的含煤地层、矿床硫化物及大气酸沉降有关。     

汉江上游庹家湾剖面化学风化特征及其意义

通过对庹家湾黄土剖面元素的分析,揭示汉江上游地区黄土中常量元素的地球化学行为及对气候变化的响应。用X-Ray荧光光谱仪、MS-2型磁化率仪分别测量了庹家湾剖面元素含量及磁化率值,实验结果显示:庹家湾剖面的风成黄土化学成分以SiO₂、Al₂O₃和Fe₂O₃为主,三者含量分别为355.83 g/kg、79.62 g/kg、62.64 g/kg;以Ti为参比,Na、Ca、Mg、Si、K表现为迁移淋失,Fe、Al元素相对富集,元素的活动性及其迁移序列为Na > Ca > Mg > Si > K > Al > Fe;Fe和Al元素含量曲线及CIA曲线在228~260 cm（L₁~S₁）和294~370 cm（L₁~S₂）深度处出现明显的峰值,并与磁化率曲线呈现出高度一致性,指示该层化学风化程度明显高于典型黄土,接近于古土壤（S₀）,OSL年龄在27.5~21.5 ka B.P.之间。庹家湾剖面常量元素在剖面的变化,表明在汉江上游地区晚更新世末期的气候并非是持续干燥寒冷,而是存在一定的气候波动,在27.5~21.5 ka B.P.期间风化成壤作用较为明显,气候相对温暖湿润。     

广西某地花岗岩风化壳中稀土元素特征与iREE矿床成矿机制

为了解风化壳中离子交换相稀土元素的特征,对广西某地花岗岩风化壳剖面样品进行了X射线衍射及主量、稀土元素地球化学特征的研究.剖面自上而下可划分为腐殖土层(A₁)、亚粘土层(A₂)、网纹状风化层(B₁)和全风化层(B₂);自A₁至B₂,粘土矿物的含量和化学风化蚀变指数快速降低;与母岩相比A₁、A₂、B₁中全相Ce、Nd和HREE相对富集,B₂中全相稀土与母岩特征相似,所有样品的离子交换相HREE亏损,Y相对富集;离子交换相轻、重稀土一起富集在B₂中.据此推测,花岗岩中褐帘石、榍石等易风化的稀土矿物为离子交换相稀土提供了主要的物源,锆石、磷钇矿等难风化的稀土矿物的残留及表生稀土矿物的形成使全相HREE相对富集;离子交换相轻、重稀土元素的分馏程度随风化程度的增加而变化.      

Chemical Weathering and CO2 Consumption of Chishuihe River Basin,Guizhou Province

Carbon sink produced during rock weathering is critical to global carbon cycles. In this work, we analyzed the major ion chemistry of the Chishuihe River Basin, and the major ion composition of the Chishuihe River system and the principal component analysis was applied for estimating the weathering rate and atmospheric CO₂ consumption via the rock chemical weathering. The results demonstrated that the chemical composition of the river was dominated by Ca²⁺, Mg²⁺, HC and S. The average concentration(317.88 mg/L) of the total dissolved solids within the Chishuihe River was higher than the average value (65 mg/L) of world rivers. The Gibbs graph combining major ion element ratio analysis indicated that the catchment major ion composition mainly originated from rock weathering, primarily from carbonate weathering, sparsely from silicate weathering. Carbonate and silicate weathering contributed 70.77% and 5.03% separately to the dissolved loads. The anthropogenic and precipitation impact was limited. According to calculation based on principal component and the ion composition characteristics, the chemical weathering rate was 126.716 t/(km²·a), significantly higher than that of the Yellow River and Yangtze River, and also higher than the average rate of the global major rivers. The CO₂ consumption flux based on annual average runoff was 10.96×10⁹ mol/a, and the CO₂ consumption rate by chemical weathering was 5.79×10⁵ mol/(km²·a).     

The Influence of Allogenic Water and Sulfuric Acid to Karst Carbon Sink in Karst Subterranean River in Southern Hu’nan

Evaluating the impact of allogenic water and sulfuric acid on karst carbon sink not only helps to improve the accurate calculation of soil CO₂ uptake by rock weathering, but also obtains a complete understanding of the global carbon cycle. Groundwater samples were collected from four karst subterranean rivers watershed within different lithology strata in Wushui Basin, upstream of Beijiang Basin, Hunan Province, for revealing the important impact of silicate weathering on hydrochemistry of groundwater. To estimate the contribution of soil CO₂ uptake by silicate weathering to CO₂ uptake by rock weathering, the Galy model was employed in this article. The important impact of sulfuric acid on CO₂ uptake by carbonate weathering resulting from the substitution of carbonic acid by protons from sulfuric acid was investigated. Our results showed that the concentration of Na⁺, K⁺ and SiO₂ in L01,L02 subterranean river with silicate strata in watershed were higher than that in L03,L04 subterranean river without silicate strata in watershed, which implied that the contribution of silicate weathering to Na⁺,K⁺ and SiO₂ was very important in watershed within silicate strata . The changeable equivalent ratio between (Ca²⁺+Mg²⁺) and HCO₃^- was 1.05 to 1.15, and the value of [Ca²⁺+Mg²⁺]/[HCO₃^-+SO₄^2-] was 0.99 to 1.08. The concentrations of Ca²⁺ and Mg²⁺ exceeded the equivalent concentrations of HC₃^-, and the excess of Ca²⁺ and Mg²⁺cations were compensated by SO₄^2-, which suggested that sulfuric acid has an important influence on carbonate dissolution. The contribution of soil CO₂uptake by silicate weathering to CO₂ consumption in L01 and L02 subterranean river were 3.36% and 2.22%, respectively, whereas the contribution in L03, L04 subterranean river were less than 0.50%, indicating that the contribution of soil CO₂ uptake by silicate weathering was important in the subterranean river basin within silicate strata. Due to the contributions made by sulfuric acid, the CO₂ consumption in four subterranean rivers decreased by 4.84%, 4.52%, 6.20%, 9.36%, respectively.     

Chemical mass balance of calcrete genesis on the Toledo granite (Spain)

The chemical mass balance of calcrete genesis is studied on a typical sequence developed in granite, in the Toledo mountains, Central Spain.

Field evidence and petrographic observations indicate that the texture and the bulk volume of the parent rock are strictly preserved all along the studied calcrete profile.

Microscopic observations indicate that the calcitization process starts within the saprolite, superimposed on the usual mechanisms of granite weathering: the fresh rock is first weathered to secondary clays, mainly smectites, which are then pseudomorphically replaced by calcite. Based on this evidence, chemical mass transfers are calculated, assuming iso-volume transformation from the parent rock to the calcrete.

The mass balance results show the increasing loss of matter due to weathering of the primary phases, from the saprolite towards the calcrete layers higher in the sequence. Zr, Ti or Th, which are classically considered as immobile during weathering, are also depleted along the profile, especially in the calcrete layer. This results from the prevailing highly alkaline conditions, which could account for the simultaneous precipitation of CaCO₃ and silicate dissolution.

The calculated budget suggests that the elements exported from the weathering profile are provided dominantly by the weathering of plagioclase and biotite. We calculate that 8–42% of the original Ca remains in granitic relics, while only 15% of the authigenic Ca released by weathering is reincorporated in the calcite. This suggests that 373 kg/m² of calcium (i.e., three times the original amount) is imported into the calcrete from allochtonous sources, probably due to aeolian transport from distant limestone formations.     

华北克拉通北部14亿年前古风化壳的地球化学特征及古气候意义

最近在河北涞水紫石口剖面的中元古界铁岭组-下马岭组界线(~1400 Ma)附近,发现了一套保存完好的碳酸盐岩古风化壳。野外观测和岩石矿物学研究显示,该古风化壳的岩性以含褐铁矿的铁质泥岩和伊利石粘土岩为主。相对上地壳平均化学组分来说,古风化壳的主量元素氧化物中Al₂O₃、TiO₂、Fe₂O₃和K₂O富集,SiO₂轻微亏损,CaO、MgO和Na₂O强烈亏损;微量元素中Sc、Cu、Ga、Zr、Hf、Nb、Ta、In、Th、Li、V、Zn、Rb、Cs、W、Pb、U、Mo和Bi等化学性质稳定或易吸附于粘土中的元素含量接近上地壳平均化学组分值或呈不同程度富集,易溶元素Cr、Sr和Cd相对亏损;稀土元素全部富集,轻稀土元素与重稀土元素分异程度大,Ce正异常,Eu负异常。U/Th、Fe³⁺/Fe²⁺、Ce的正异常、化学蚀变指数与风化淋滤指数等多种指标都指示该古风化壳是在强氧化的中等到强烈的风化淋滤程度条件下形成。上述研究表明,在中元古代盖层纪(Calymmian Period,1600~1400 Ma)与延展纪(Ectasian Period,1400~1200 Ma)之交,剖面所在的华北克拉通北部应属于温暖湿润的热带-亚热带气候。     

Tropical chemical weathering of hillslope deposits and bedrock source in the Aburrá Valley, northern Colombian Andes

Tropical chemical weathering produces extensive lateritization and formation of deep weathering profiles. Both processes are fundamental to landscape evolution and slope instability. The Aburrá Valley of the northern Colombian Andes is characterized by tropical conditions. The valley slopes are mostly covered by hillslope deposits originating from four basement rock suites which comprise contrasting granitoid, volcanic–sedimentary, ophiolitic, and metamorphic sources, respectively. Tropical chemical weathering of the Aburrá hillslope deposits and their respective bedrock were examined using X-ray fluorescence and X-ray diffraction analysis, to document and quantify their chemical weathering profiles, compositions, and mineralogical properties. The Chemical Index of Alteration (CIA), loss on ignition (LOI), and the Mobiles index (Imob) were used to quantify the degree of weathering of hillslope deposits and bedrock source. Weathering trends were analyzed using A–CN–K and A–CNK–FM diagrams. The material mantling the slopes in the Aburrá Valley records an intense weathering history. Chemical weathering is characterized by increased development of clay minerals (kaolinite, halloysite) and iron and aluminum sesquioxides. Lateritization characterizes the final stage of the weathering profiles. Concentrations of CaO, Na₂O, K₂O decrease markedly in the weathering products compared to the fresh bedrock source, whereas concentrations of Al₂O₃, Fe₂O₃, and MgO increase significantly. CIA ratios of matrix slope deposits derived from all four sources near 100, whereas those of boulder slope deposits and saprolites are lower, but exceed source rock values. Different A–CN–K weathering paths are evident for each lithotype, validating the correlation established between the hillslope deposits and their various parents. Chemical weathering indices in some samples are strongly influenced by the presence of sesquioxides, as reflected by high LOI, anomalously low CIA, and varying enrichment trends on the A–CNK–FM diagrams. Consequently, different chemical indices based on different criteria need to be combined to obtain best results, as illustrated here by the combination of LOI, CIA, and Imob. The overall results suggest that tropical conditions have dominated for a long time in the northern Colombian Andes, leading to uniformly high weathering indices in matrix slope deposits irrespective of parent lithotype. Prolonged warm and humid conditions could thus be responsible for the weathering and remobilization of extensive old hillslope deposits during the Quaternary. However, in addition to the influence of climatic factors, tectonism has also undoubtedly influenced slope evolution in the Aburrá Valley.     

中国东部玄武岩风化土壤的粘土矿物及碳 汇地球化学研究

     玄武岩风化是大气CO2 的一个主要碳汇过程,气候条件是影响玄武岩风化和固碳速率的重要因素。该文选择中国东 部不同气候带的新生代玄武岩典型风化剖面,进行了粘土矿物和常量元素分析。结果显示,在内蒙古-海南岛的采样区间内, 随着气候条件由干冷向暖湿转化,风化剖面中粘土矿物组合呈现蒙脱石+ 伊利石+ 高岭石→蒙脱石+ 高岭石→高岭石+ 三水 铝石的转变。剖面中土壤元素得失状况也显示出相应的规律,由于存在粉尘输入与风化淋滤作用的综合影响,在干冷的内 蒙古地区,粉尘对于Ca,K,Na,Si 等元素的输入量大于这些元素的淋失量;在山东地区,Ca,K,Na 元素开始快速淋失, 大于粉尘的输入量;在苏皖地区,Si 元素的淋失量开始小于粉尘输入量;而在湿热的海南地区,风化作用强烈,Si显示出 大量淋失的特点,碱性元素几乎全部流失。根据元素的相对得失率和北方粉尘平均组分的校正,初步估算了研究区内玄武 岩风化对大气CO2 的消耗速率,其数值在5.37~181.00 t（km2·a）之间,与Dessert 等（2003）的研究结果大致相当。