Insights into combined radiogenic and stable strontium isotopes as tracers for weathering processes in subglacial environments
Insights into combined radiogenic and stable strontium isotopes as tracers for weathering processes in subglacial environments
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DOI：
10.1016/j.chemgeo.2016.03.008
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发表时间：
2016-07
期刊：
影响因子：
3.9
通讯作者：
E. Stevenson;S. Aciego;P. Chutcharavan;I. Parkinson;K. Burton;M. Blakowski;C. Arendt
中科院分区：
文献类型：
--
作者：
E. Stevenson;S. Aciego;P. Chutcharavan;I. Parkinson;K. Burton;M. Blakowski;C. Arendt
本研究报告了一个融化季节柠檬溪冰川（阿拉斯加朱诺冰原）冰下流出的溶解负荷和悬浮沉积物中稳定的放射性同位素行为。现场测量（放电、总碱度、pH值和电导率）与元素浓度、x射线衍射（XRD）分析和放射性锶同位素测量相结合，以解释稳定锶同位素比中观察到的变化。稳定Sr同位素组成（88Sr/86Sr比值δ88/86Sr，‰）平均为0.31±0.05‰，比悬浮物重0.18±0.03‰，比局部基岩重0.20 ~ 0.26‰。我们将溶解负荷中较重同位素的富集归因于次生风化矿物对较轻Sr同位素的吸收，使溶解负荷变得更重。x射线衍射（XRD）分析证实了悬浮沉积物中存在粘土，热力学模型表明存在氧化铁相。虽然不能完全排除原生矿物的溶解对Sr同位素组成的控制作用，但我们的数据表明，次生矿物的形成程度可能起着重要作用。黑云母等矿物的优先风化作用（与悬浮沉积物中发现的矿物组合一致），以及来自偏碳酸盐（来自育空-塔纳纳地形）的放射性成因方解石的潜在存在，可能导致溶解负荷87sr /86Sr的小季节变化，从87sr /86Sr(DL)= 0.71048到0.710647。利用稳定锶同位素和放射性锶同位素的组合研究风化过程表明，放射性锶同位素提供了有关原生相风化的信息。而稳定的Sr同位素数据似乎记录了有关次生矿物形成程度的信息，其中次生矿物与轻同位素结合，将溶解负荷驱动到重值。
This study reports stable and radiogenic strontium isotope behaviour in the dissolved load and suspended sediments from the subglacial outflow of the Lemon Creek glacier (Juneau Ice Field, Alaska) over a single melt season.In situmeasurements (discharge, total alkalinity, pH and conductivity) are combined with elemental concentrations, X-ray diffraction (XRD) analysis and radiogenic strontium isotope measurements to interpret the variations observed in stable strontium isotopic ratios.The stable Sr isotope composition (88Sr/86Sr ratio expressed as δ88/86Sr, ‰) of the dissolved load averages 0.31 ± 0.05‰, and is heavier than both the suspended sediment 0.18 ± 0.03‰, as well as local bedrocks ~ 0.20 to 0.26‰. We attribute the enrichment of heavier isotopes in the dissolved load to the uptake of lighter Sr isotopes by secondary weathering minerals, driving the dissolved load to heavier values. X-ray diffraction (XRD) analysis confirms the presence of clays in the suspended sediments and thermodynamic modelling suggests the presence of iron oxy-hydroxide phases. Although it is not possible to completely rule out the effect of dissolution of primary minerals in controlling Sr isotopic compositions of the dissolved load, our data indicate that the extent of secondary mineral formation likely plays a significant role. The preferential weathering of minerals such as biotite (consistent with the mineralogical assemblages found in the suspended sediments), as well as the potential presence of radiogenic calcites from metacarbonates (derived from the Yukon-Tanana terrain), may be driving the small seasonal shifts in87Sr/86Sr of the dissolved load to more radiogenic compositions, from87Sr/86Sr(DL)= 0.71048 to 0.710647.Using the combination of stable and radiogenic strontium isotopes to investigate weathering processes shows that radiogenic Sr isotopes provide information regarding weathering of primary phases. While the stable Sr isotope data appear to record information regarding the extent of secondary mineral formation, where secondary minerals incorporate the light isotopes, driving the dissolved load to heavy values.