Age of river basins in Guadeloupe impacting chemical weathering rates and land use

The Lesser Antilles have very high chemical weathering rates, with values that can reach 1290 t/km²/a. The tropical environment induces high precipitation rates, high temperature, dense vegetation, with sharp relief and thick soils. Because of volcanic activity, frequent pyroclastic flows produce very erodible and porous materials. In addition, agriculture induces important land use changes which replace existing native forest cover with banana and sugar cane plantations. Their surface can cover as much as 40% of the total area of a river basin. The aim of this study is to identify key parameters, either natural or anthropogenic, that control chemical weathering rates. Among the combined impact of all parameters (climate, runoff, slopes, vegetation etc.), basin age seems to be the control parameter: the younger the basin, the higher the weathering rate. A correlation between the chemical weathering rate and the basin age suggests that young volcanic rocks are more easily weathered than old ones: young fresh material is easily mobilized by erosion, while for older rocks with thick soil covers, chemical rates are much lower. A combined effect between the higher erodibility and a higher climate erosivity of the younger relief could be observed. Moreover, a correlation between banana plantations and the chemical weathering rates that can be explained by an increase of infiltration, due to stem flow processes is shown here. Banana plantations also have a correlation with the basin age, older basins being more favorable terrains for cultivation.     

Mechanisms for chemostatic behavior in catchments: Implications for CO2 consumption by mineral weathering

Concentrations of weathering products in streams often show relatively little variation compared to changes in discharge, both at event and annual scales. In this study, several hypothesized mechanisms for this “chemostatic behavior” were evaluated, and the potential for those mechanisms to influence relations between climate, weathering fluxes, and CO₂ consumption via mineral weathering was assessed. Data from Loch Vale, an alpine catchment in the Colorado Rocky Mountains, indicates that cation exchange and seasonal precipitation and dissolution of amorphous or poorly crystalline aluminosilicates are important processes that help regulate solute concentrations in the stream; however, those processes have no direct effect on CO₂ consumption in catchments. Hydrograph separation analyses indicate that old water stored in the subsurface over the winter accounts for about one-quarter of annual streamflow, and almost one-half of annual fluxes of Na and SiO₂ in the stream; thus, flushing of old water by new water (snowmelt) is an important component of chemostatic behavior. Hydrologic flushing of subsurface materials further induces chemostatic behavior by reducing mineral saturation indices and increasing reactive mineral surface area, which stimulate mineral weathering rates. CO₂ consumption by carbonic acid mediated mineral weathering was quantified using mass-balance calculations; results indicated that silicate mineral weathering was responsible for approximately two-thirds of annual CO₂ consumption, and carbonate weathering was responsible for the remaining one-third. CO₂ consumption was strongly dependent on annual precipitation and temperature; these relations were captured in a simple statistical model that accounted for 71% of the annual variation in CO₂ consumption via mineral weathering in Loch Vale.     

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。     

Chemical weathering and carbon dioxide consumption in a small tropical river catchment,southwestern India

Aquatic Geochemistry - Studies done on small tropical west-flowing river catchments located in the Western Ghats in southwestern India have suggested very intense chemical weathering rates and...     

The short term climatic sensitivity of carbonate and silicate weathering fluxes: Insight from seasonal variations in river chemistry

Large seasonal variations in the dissolved load of the headwater tributaries of the Marsyandi river (Nepal Himalaya) for major cations and ⁸⁷Sr/⁸⁶Sr ratios are interpreted to result from a greater dissolution of carbonate relative to silicate at high runoff. There is up to a 0.003 decrease in strontium isotope ratios and a factor of 3 reduction in the Si(OH)₄/Ca ratio during the monsoon. These variations, in small rivers sampling uniform lithologies, result from a different response of carbonate and silicate mineral dissolution to climatic forcing. Similar trends are observed in compiled literature data, from both Indian and Nepalese Himalayan rivers. Carbonate weathering is more sensitive to monsoonal runoff because of its faster dissolution kinetics. Silicate weathering increases relative to carbonate during the dry season, and may be more predominant in groundwater with longer water-rock interaction times. Despite this kinetic effect, silicate weathering fluxes are dominated by the monsoon flux, when between 50% and 70% of total annual silicate weathering flux occurs.     

Chemical weathering, river geochemistry and atmospheric carbon fluxes from volcanic and ultramafic regions on Luzon Island, the Philippines

We investigated rates of chemical weathering of volcanic and ophiolitic rocks on Luzon Island, the Philippines. Luzon has a tropical climate and is volcanically and tectonically very active, all factors that should enhance chemical weathering. Seventy-five rivers and streams (10 draining ophiolites, 65 draining volcanic bedrock) and two volcanic hot springs were sampled and analyzed for major elements, alkalinity and ⁸⁷Sr/⁸⁶Sr. Cationic fluxes from the volcanic basins are dominated by Ca²⁺ and Mg²⁺ and dissolved silica concentrations are high (500-1900 μM). Silica concentrations in streams draining ophiolites are lower (400-900 μM), and the cationic charge is mostly Mg²⁺. The areally weighted average CO₂ export flux from our study area is 3.89 ± 0.21 × 10⁶ mol/km²/yr, or 5.99 ± 0.64 × 10⁶ mol/km²/yr from ophiolites and 3.58 ± 0.23 × 10⁶ mol/km²/yr from volcanic areas (uncertainty given as ±1 standard error, s.e.). This is ∼6-10 times higher than the current best estimate of areally averaged global CO₂ export by basalt chemical weathering and ∼2-3 times higher than the current best estimate of CO₂ export by basalt chemical weathering in the tropics. Extrapolating our findings to all tropical arcs, we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments, which amount to ∼1% of the global exorheic drainage area. Chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long-term carbon cycle.     

青藏高原东部长江流域盆地地表化学剥蚀通量剥蚀速率大气CO2净消耗率研究

2000~2002年期间,笔者对青藏高原东部长江流域溶质载荷分别进行了取样分析并对流域盆地化学剥蚀通量、剥蚀速率和大气CO₂净消耗率进行了计算。结果表明,流域盆地化学剥蚀速率以河源区楚玛尔河最高为2.34×10⁶mol/a/km²,沱沱河最低为1.40×10⁶mol/a/km²,四大支流雅砻江为1.69×10⁶mol/a/km²,金沙江为1.74×10⁶mol/a/km²,大渡河为1.57×10⁶mol/a/km²,岷江为1.88×10⁶mol/a/km²;流域盆地ФCO₂估算结果以大渡河最高为101.81×10³mol/a/km²,楚玛尔河最低为7.55×10³mol/a/km²,金沙江为44.38×10³mol/a/km²,雅砻江为69.64×10³mol/a/km²,岷江为81.90×10³mol/a/km²,沱沱河为21.90×10³mol/a/km²。并对长江流域地表化学剥蚀速率主要控制因素进行了讨论。     

Solute generation and CO2 consumption during silicate weathering under subtropical,humid climate,northern Okinawa Island,Japan

The chemical characteristics of freshwaters draining the silicate rocks in the northern part of Okinawa Island were studied to understand solute generation processes, and to determine rates of chemical weathering and CO₂ consumption. It was observed that the water chemistry is highly influenced by marine aerosols, contributing more than 60% of total solute. Significant positive correlations observed for chloride versus dissolved silica and chloride versus bicarbonate suggest a strong influence of evapotranspiration on the seasonality of solute concentration. It was also found that chemical weathering has been highly advanced in which the dominant kaolinite minerals are being gibbsitized. Carbonic acid was found to be the major chemical weathering agent, releasing greater than 80% of weathering-derived dissolved cations and silica while the remaining portion was attributed to weathering by sulfuric acid generated via oxidation of pyrite contained in the rocks. The flux of basic cations, weathering-derived silica and CO₂ consumption were relatively high due to favourable climatic condition, topography and high rate of mechanical erosion. Silicate weathering rates for basic cations were estimated to be 6.7–9.7 ton km^− 2 y^− 1. Carbon dioxide consumed by silicate weathering was 334–471 kmol km^− 2 y^− 1 which was slightly higher than that consumed by carbonate weathering. In general, divalent cations (Mg and Ca) and bicarbonate alkalinity derived from carbonate dissolution were higher than those from silicate weathering. As a consequence, the evolution of chemical species in the freshwaters of northern area of Okinawa Island to a large extent could be explained by mixing of two components, characterized by waters with Na⁺ and Cl⁻ as predominant species and waters enriched with Ca²⁺ and HCO₃⁻.     

Climate change detection and attribution in the Ganga-Brahmaputra-Meghna river basins

Ganga-Brahmaputra-Meghna(GBM) river basin is the third-largest and one of the most populated river basins in the world. As climate change is affecting most of the hydrometeorological variables across the globe, this study investigated the existence of climate change signal in all four climatological seasons in the GBM river basin and assessed the contribution of anthropogenic activities, i.e., Greenhouse Gases(GHGs) emission in the change. Significant decreasing trends in the monsoon and a small increase in pre-monsoon precipitation were observed. Negligible change was detected in post-monsoon and winter season precipitation. CMIP5 GCMs were used for climate change detection, change point estimation, and attribution studies. Support Vector Machine(SVM) regression method was adopted to downscale GCM variables at the local scale. Monte-Carlo simulation approach was used to detect changes in different seasons. The climate change ‘signals' were detectable after the year 1980 using Signal to Noise ratio(SNR)method in the majority of central and north-western regions. The change point was detectable only in annual monsoon precipitation at the basin level. Attribution analysis indicated 50% contribution of anthropogenic activities(GHGs) to annual monsoon precipitation changes. So, there is high confidence that monsoon precipitation in GBM has significantly changed due to anthropogenic activities. Different mitigation and adaption measures are also suggested, which may be adopted to manage the growing demand and water availability in the basin.     

Seasonally chemical weathering and CO<Subscript>2</Subscript> consumption flux of Lake Qinghai river system in the northeastern Tibetan Plateau

The major cation and anion compositions of waters from the Lake Qinghai river system (LQRS) in the northeastern Tibetan Plateau were measured. The waters were collected seasonally from five main rivers during pre-monsoon (late May), monsoon (late July), and post-monsoon (middle October). The LQRS waters are all very alkaline and have high concentrations of TDS (total dissolved solids) compared to rivers draining the Himalayas and the southeastern Tibetan Plateau. Seasonal variations in the water chemistry show that, except the Daotang River, the TDS concentration is high in October and low in July in the LQRS waters. The forward models were used to quantify the input of three main rivers (Buha River, Shaliu River, and Hargai River) from rain, halite, carbonates, and silicates. The results suggest that (1) atmospheric input is the first important source for the waters of the Buha River and the Shaliu River, contributing 36–57% of the total dissolved cations, (2) carbonate weathering input and atmospheric input have equal contribution to the Hargai River water, (3) carbonate weathering has higher contribution to these rivers than silicate weathering, and (4) halite is also important source for the Buha River. The Daotang River water is dominated by halite input owing to its underlying old lacustrine sediments. The water compositions of the Heima River are controlled by carbonate weathering and rainfall input in monsoon season, and groundwater input may be important in pre-monsoon and post-monsoon seasons. After being corrected the atmospheric input, average CO₂ drawdown via silicate weathering in the LQRS is 35 × 10³ mol/km² per year, with highest in monsoon season, lower than Himalayas and periphery of Tibetan Plateau rivers but higher than some rivers draining shields.     

珠江流域碳酸盐岩与硅酸盐岩风化对大气CO2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。     

Contribution of carbonate rock weathering to the atmospheric CO2 sink

To accurately predict future CO₂ levels in the atmosphere, which is crucial in predicting global climate change, the sources and sinks of the atmospheric CO₂ and their change over time must be determined. In this paper, some typical cases are examined using published and unpublished data. Firstly, the sensitivity of carbonate rock weathering (including the effects by both dissolution and reprecipitation of carbonate) to the change of soil CO₂ and runoff will be discussed, and then the net amount of CO₂ removed from the atmosphere in the carbonate rock areas of mainland China and the world will be determined by the hydrochem-discharge and carbonate-rock-tablet methods, to obtain an estimate of the contribution of carbonate rock weathering to the atmospheric CO₂ sink. These contributions are about 0.018 billion metric tons of carbon/a and 0.11 billion metric tons of carbon/a for China and the world, respectively. Further, by the DBL (Diffusion Boundary Layer)-model calculation, the potential CO₂ sink by carbonate rock dissolution is estimated to be 0.41 billion metric tons of carbon/a for the world. Therefore, the potential CO₂ source by carbonate reprecipitation is 0.3 billion metric tons of carbon/a. Received: 12 May 1999 · Accepted: 16 August 1999     

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。     

秋季黄河pCO2控制因素及水-气界面通量

根据2006年11月1~10日,秋季黄河平水期二氧化碳分压(pCO₂)的现场实测数据及相关同步观测资料,对黄河表层水pCO₂的分布及其影响因素进行了研究。结果表明:水体pCO₂在80~166Pa,平均值110Pa,在世界主要河流中属中等偏下水平;空间分布存在较大的不均匀性,中游高于上游和下游。浮游植物的光合作用对pCO₂有一定的影响但强度较弱,即使在叶绿素最高值3.58μg/L的包头站pCO₂仍达到91Pa。黄河水体有机物含量较低且继承了陆源有机物难降解的特性,干流和库区EpCO₂/AOU的比值为0.14和0.20,远低于生物好氧呼吸作用控制水体pCO₂的理论下限0.62,因此,生物好氧呼吸作用对水体pCO₂的贡献不大。悬浮物(TSS)含量为3.77~1308mg/L,溶解无机碳(DIC)含量为3.03~4.14mmol/L,普遍高于世界其它河流且最大值均出现在潼关站;同时水体pCO₂与TSS、PIC、DIC含量具有极好的正相关性。因此黄河流域强烈的机械侵蚀和化学风化作用形成的碳酸盐体系是控制水体pCO₂的主要因素。利用Wanninkhof提出的淡水水-气交换系数的通量模式估算,黄河水域水-气界面CO₂交换速率约为0.229μmol/m2·s,秋季可向大气释放CO₂14.5亿moL,相当于8250km2草原或是112km2森林一年的固碳量。黄河CO₂释放通量与渥太华河相近,但要远小于亚马逊河。     

Slope-control on the aspect ratio of river basins

Although it is traditionally thought that drainage basins are geometrically similar, we present new results which indicate that the aspect ratio of weakly dissected river basins at large scales (10–10³ km²) depends on the surface slope: steeper surfaces develop narrower and lengthier basins whereas more gently dipping surfaces develop more equant and wider basins. This relationship is interpreted to be related to the nature of water flow over rough surfaces, with steeper slopes causing less flow convergence and lengthier and narrower basins. We derive an empirical relationship that can be used to infer the slope of a surface on which a river basin acquired its geometry based solely on a measure of its basin form. This relation provides a unique means of inferring the relative chronology of river basin development with respect to surface tilting and therefore provides a direct link between river basin morphology and tectonics.     

河西内陆河地区径流模型概述

径流模拟是河西内陆河地区水文研究中最基本、最重要的一个环节,也是研究其它水文问题的基础.目前广泛应用在河西地区的径流模拟模型大致分为系统理论模型、概念模型和分布式模型三类.但是这些模型各自有哪些特点,应用中应注意哪些问题,却缺少宏观的认识.总结了这三类模型在河西内流河区应用的代表性研究成果,并结合当前所应用模型存在的问题,探讨了研究区未来水文模型可能的发展方向.     

天星河、文峰河流域岩溶水化学及影响因素

重庆东北部地区巫溪天星河、文峰河的岩溶流域水化学特征研究结果表明:研究区泉水样品的p H值为6.47~8.28,平均值为7.79;流域河水饱和指数(SIc)的平均值为0.14,表明其变化受到流域碳酸盐岩分布的控制。水化学类型以HCO3-Ca型水为主,Ca2+和HCO-3平均分别占阳、阴离子的75%和82%。通过分析Gibbs图,发现泉水的主要组分属于"岩石风化类型",主要反映了岩石的溶解作用对泉水水化学的影响。流域内部分水样NO-3和SO2-4较高,可能是受工农业生产等人类活动的影响引起,此外硫酸也参与了碳酸盐矿物的溶解。