Quantifying chemical weathering rates along a precipitation gradient on Basse-Terre Island, French Guadeloupe: New insight from U-series isotopes in weathering rinds
Quantifying chemical weathering rates along a precipitation gradient on Basse-Terre Island, French Guadeloupe: New insight from U-series isotopes in weathering rinds
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量化法属瓜德罗普岛巴斯特尔岛沿降水梯度的化学风化率：风化皮中 U 系列同位素的新见解
DOI：
10.1016/j.gca.2016.08.040
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发表时间：
2016
影响因子：
5
通讯作者：
Brantley, Susan L.
中科院分区：
文献类型：
--
作者：
Engel, Jacqueline M.;Ma, Lin;Sak, Peter B.;Gaillardet, Jerome;Ren, Minghua;Engle, Mark A.;Brantley, Susan L.
在土壤和腐泥土内部，岩石碎片可以形成风化碎屑（围绕未风化核心的蚀变外皮），这些风化外皮为研究风化的起始和长期风化速率提供了一个极好的现场系统。最近，铀系（U系列）不平衡在确定风化外皮形成速率和量化控制风化外皮风化进展速率的因素方面显示出巨大的潜力。为了进一步研究 U 系列同位素技术是否可以记录长期风化率随降水变化的差异，我们对小安的列斯群岛的热带火山巴斯特尔岛进行了一项新的风化皮研究。在这项研究中，我们首次描述了陡峭降水梯度中多个风化层的风化反应并量化了风化进展速率。在德赛流域的两个风化碎屑中，对年平均降水量 (MAP) = 1800 毫米、温度 (MAT) = 23 °C 的两个风化碎屑进行了电子微探针 (EMP) 点测量、大量主元素含量和 U 系列同位素组成的测定。在这些碎屑上，针对外皮-核心边界具有不同曲率（高、中和低）的位置测量了五个核心-外皮横断面。结果表明，在外皮形成过程中，元素损失的比例按以下顺序降低：Ca ≈ Na > K ≈ Mg > Si ≈ Al > Zr ≈ Ti ≈ Fe。这些观察结果与风化开始后的反应顺序一致：具体来说，玻璃基质和初级矿物（斜长石、辉石）风化产生氢氧化铁、三水铝石和少量高岭石。由于含U的土壤孔隙水以溶解的U相形式渗透到外皮中，铀在外皮中显示出添加曲线。然后，随着铁铝氧化物的沉淀，铀并入果皮中。这些过程导致果皮中显着的 U 系列同位素不平衡。这是首次对同一流域的多个风化碎屑进行 U 系列同位素不平衡分析，并显示出一致的结果。 U 系列不平衡允许使用 U 系列质量平衡模型确定外皮形成年龄和风化提前率。风化提前率通常随着曲率的减小而降低：高曲率位置为～0.17±0.10mm/kyr，中等曲率位置为～0.12±0.05mm/kyr，低曲率位置为～0.11±0.04、0.08±0.03、0.06±0.03mm/kyr。风化模型的预测很好地支持了观察到的曲率和风化速率之间的正相关性，即外皮-核边界的曲率控制了碎屑尺度上的孔隙度产生和风化推进速率。在分水岭尺度上，新的风化推进速率是根据相对干燥的德赛流域的低曲率样带得出的（平均速率为 0.08 mm/kyr；MAP = 1800 mm 和MAT = 23 °C）比先前在巴斯特尔岛更湿润的布拉斯大卫流域（〜0.18 mm/kyr，低曲率横断面；MAP = 3400 mm 和 MAT = 23 °C）中确定的果皮形成速率慢约 60%。因此，MAP 的加倍大致与风化进展速率的加倍相关。新的外皮研究强调了 100 kyr 时间范围内降水对风化率的影响。因此，风化皮是研究跨环境梯度的长期化学风化的合适系统，补充了短期河流溶质通量。
Inside soil and saprolite, rock fragments can form weathering clasts (alteration rinds surrounding an unweathered core) and these weathering rinds provide an excellent field system for investigating the initiation of weathering and long term weathering rates. Recently, uranium-series (U-series) disequilibria have shown great potential for determining rind formation rates and quantifying factors controlling weathering advance rates in weathering rinds. To further investigate whether the U-series isotope technique can document differences in long term weathering rates as a function of precipitation, we conducted a new weathering rind study on tropical volcanic Basse-Terre Island in the Lesser Antilles Archipelago. In this study, for the first time we characterized weathering reactions and quantified weathering advance rates in multiple weathering rinds across a steep precipitation gradient. Electron microprobe (EMP) point measurements, bulk major element contents, and U-series isotope compositions were determined in two weathering clasts from the Deshaies watershed with mean annual precipitation (MAP) = 1800 mm and temperature (MAT) = 23 °C. On these clasts, five core-rind transects were measured for locations with different curvature (high, medium, and low) of the rind-core boundary. Results reveal that during rind formation the fraction of elemental loss decreases in the order: Ca ≈ Na > K ≈ Mg > Si ≈ Al > Zr ≈ Ti ≈ Fe. Such observations are consistent with the sequence of reactions after the initiation of weathering: specifically, glass matrix and primary minerals (plagioclase, pyroxene) weather to produce Fe oxyhydroxides, gibbsite and minor kaolinite.Uranium shows addition profiles in the rind due to the infiltration of U-containing soil pore water into the rind as dissolved U phases. U is then incorporated into the rind as Fe-Al oxides precipitate. Such processes lead to significant U-series isotope disequilibria in the rinds. This is the first time that multiple weathering clasts from the same watershed were analyzed for U-series isotope disequlibrian and show consistent results. The U-series disequilibria allowed for the determination of rind formation ages and weathering advance rates with a U-series mass balance model. The weathering advance rates generally decreased with decreasing curvature: ∼0.17 ± 0.10 mm/kyr for high curvature, ∼0.12 ± 0.05 mm/kyr for medium curvature, and ∼0.11 ± 0.04, 0.08 ± 0.03, 0.06 ± 0.03 mm/kyr for low curvature locations. The observed positive correlation between the curvature and the weathering rates is well supported by predictions of weathering models, i.e., that the curvature of the rind-core boundary controls the porosity creation and weathering advance rates at the clast scale.At the watershed scale, the new weathering advance rates derived on the low curvature transects for the relatively dry Deshaies watershed (average rate of 0.08 mm/kyr; MAP = 1800 mm and MAT = 23 °C) are ∼60% slower than the rind formation rates previously determined in the much wetter Bras David watershed (∼0.18 mm/kyr, low curvature transect; MAP = 3400 mm and MAT = 23 °C) also on Basse-Terre Island. Thus, a doubling of MAP roughly correlates with a doubling of weathering advance rate. The new rind study highlights the effect of precipitation on weathering rates over a time scale of ∼100 kyr. Weathering rinds are thus a suitable system for investigating long-term chemical weathering across environmental gradients, complementing short-term riverine solute fluxes.
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