The Li isotope response to mountain uplift
The Li isotope response to mountain uplift
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DOI：
10.1130/g36162.1
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发表时间：
2015
期刊：
影响因子：
5.8
通讯作者：
P. V. Strandmann;G. Henderson
中科院分区：
文献类型：
--
作者：
P. V. Strandmann;G. Henderson
硅酸盐风化是大气中CO2去除的关键过程。有人提出，山脉隆升导致硅酸盐风化增加，并导致了长期的新生代冷却趋势，尽管这一假设仍然存在争议。锂同位素是硅酸盐风化过程的示踪剂，并可能使这一假设得到验证。最近的研究表明，在喜马拉雅隆升期间，海水中的Li同位素比值增加（始于约2000年）。45 Ma），但隆升与河流沃茨Li同位素比值的关系尚未得到验证。在这里，我们研究锂同位素比值在河流排水集水区与可变的隆起率从南岛，新西兰。δ 7 Li值与隆升呈负相关，表明快速隆升区δ 7 Li值较低，而较平坦的洪泛平原区δ 7 Li值较高。结合U活度比，数据表明，原生硅酸盐被搬运到洪泛区，δ 7 Li和（234 U/238 U）被驱动到高值，这是由于次生矿物优先吸收6 Li和较长的流体-矿物接触时间，使沃茨富集234 U。相反，在山区，新鲜的原生矿物表面不断提供，驱动δ 7 Li和（234 U/238 U）低。这一趋势与预期的相反，如果海洋中新生代δ 7 Li的增加是由山脉隆升直接驱动的。这些数据表明，海水δ 7 Li的增加反映了洪泛平原的形成和次生矿物的形成增加，而不是山带的风化。
Silicate weathering is a key process by which CO2 is removed from the atmosphere. It has been proposed that mountain uplift caused an increase in silicate weathering, and led to the long-term Cenozoic cooling trend, although this hypothesis remains controversial. Lithium isotopes are tracers of silicate weathering processes, and may allow this hypothesis to be tested. Recent studies have demonstrated that the Li isotope ratio in seawater increased during the period of Himalayan uplift (starting ca. 45 Ma), but the relationship between uplift and the Li isotope ratio of river waters has not been tested. Here we examine Li isotope ratios in rivers draining catchments with variable uplift rates from South Island, New Zealand. A negative trend between δ7Li and uplift shows that areas of rapid uplift have low δ7Li, whereas flatter floodplain areas have high δ7Li. Combined with U activity ratios, the data suggest that primary silicates are transported to floodplains, where δ7Li and (234U/238U) are driven to high values due to preferential uptake of 6Li by secondary minerals and long fluid-mineral contact times that enrich waters in 234U. In contrast, in mountainous areas, fresh primary mineral surfaces are continuously provided, driving δ7Li and (234U/238U) low. This trend is opposite to that expected if the increase in Cenozoic δ7Li in the oceans is driven directly by mountain uplift. These data suggest that the increase in seawater δ7Li reflects the formation of floodplains and the increased formation of secondary minerals, rather than weathering of mountain belts.