LCZO - Nutrient Fluxes - Magnesium concentrations and isotopic signatures - Bisley (2009-2011)

In order to assess the effects of critical zone processes on Mg concentrations and isotopic signatures of tropical streams, we studied a well constrained, highly weathered andesitic volcaniclastic catchment in the Luquillo Critical Zone Observatory, Puerto Rico. Our results indicate that dissolved Mg concentrations and isotope ratios in the regolith pore water are mainly controlled by rain input, with weathering inputs being more important at sites with thinner regolith (2.7–0.9 m deep) and at depth (>8 m) on a thick ridgetop regolith (∼10 m). In addition to mixing of precipitation and weathering-sourced Mg, an isotopic fractionation process is taking place between dissolved Mg and the regolith, likely during dissolution or recrystallisation of Fe(III)-(hydro)oxides under alternating redox conditions. Bulk regolith is isotopically heavier than both the bedrock and the exchangeable fraction (δ26Mgregolith-bedrock = +0.03 to +0.47‰), consistent with the preferential incorporation of heavy 26Mg into secondary minerals with some exchange of sorbed Mg with isotopically lighter pore water. Magnesium concentrations in the stream show a typical dilution behaviour during a storm event, but the [Mg] – δ26Mg pattern cannot be explained by mixing of rain and pore water; the data are best explained by a steady-state fractionation model with α = 1.00115. During baseflow the stream has δ26Mg = +0.01‰, higher than any of the water samples or the bedrock. In-situ analysis of the Mg isotopic composition of bedrock minerals points at the dissolution of Mg-rich chlorite (δ26Mg = +0.19‰) as the most likely source of this isotopically heavy Mg, with mass balance calculations indicating chlorite dissolution is also the main source of Mg to the stream. Overall, our study highlights the importance of atmospheric input of nutrients to the vegetation in tropical areas covered by thick, highly leached regolith, whereas the Mg flux and Mg isotopic signature of watershed exports are dominated by bedrock dissolution delivered to the stream through deeper, usually un-sampled critical zone pathways.

为了评估关键带过程对热带溪流中镁浓度及其同位素特征的影响，我们研究了一个位于波多黎各洛斯奇洛斯关键带观测站的、结构严密、高度风化的安山质火山碎屑沉积盆地。我们的研究结果指出，土壤孔隙水中溶解的镁浓度和同位素比值主要由雨水输入控制，而在土壤层较薄（深度为2.7–0.9米）的地点以及厚层山顶土壤（约10米）的深处，风化输入显得更为重要。除了降水和风化来源的镁混合外，溶解的镁与土壤之间正在发生同位素分馏过程，这很可能是在氧化还原条件交替下，铁（III）-（水）氧化物的溶解或再结晶过程中发生的。总体土壤层在 isotopically 更重于基岩和可交换部分（δ26Mgregolith-bedrock = +0.03至+0.47‰），这与重26Mg优先进入次生矿物以及与同位素较轻的孔隙水交换吸附的镁有关。在风暴事件期间，溪流中的镁浓度表现出典型的稀释行为，但[Mg]-δ26Mg模式不能仅通过雨水和孔隙水的混合来解释；数据最佳解释为α = 1.00115的稳态分馏模型。在基流期间，溪流的δ26Mg = +0.01‰，高于任何水样或基岩。对基岩矿物的镁同位素组成的原位分析表明，富含镁的绿泥石（δ26Mg = +0.19‰）的溶解是最可能的原因，质量平衡计算也表明绿泥石的溶解也是溪流中镁的主要来源。总体而言，我们的研究强调了大气输入营养素对热带地区厚层、高度淋溶的土壤植被的重要性，而镁通量和镁同位素特征的水系输出则主要由基岩溶解通过较深、通常未采样的关键带途径输送到溪流中。