Iodine ages of pore waters at Hydrate Ridge (ODP Leg 204), Cascadia Margin: Implications for sources of methane in gas hydrates
Iodine ages of pore waters at Hydrate Ridge (ODP Leg 204), Cascadia Margin: Implications for sources of methane in gas hydrates
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DOI：
10.1016/j.epsl.2007.12.015
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发表时间：
2008-03
影响因子：
5.3
通讯作者：
Zunli Lu;H. Tomaru;U. Fehn
中科院分区：
文献类型：
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作者：
Zunli Lu;H. Tomaru;U. Fehn
美国俄勒冈州海岸外的海伯利斯海岭是活动边缘环境中天然气水合物存在的一个主要例子。它是卡斯卡迪亚边缘的一部分，也是大洋钻探计划（ODP）第204航次的重点，该航次成功地从海脊南部的九个地点回收了流体。与天然气水合物相关的孔隙流体中的碘化物浓度强烈增强，与海水相比高达5000倍，这允许使用这种亲生物元素作为有机源区的示踪剂。我们应用宇宙成因同位素129 I（T1/2=15.7 Ma）系统，以确定负责碘化物富集的有机源形成的年龄。在大洋钻探计划204航段的所有地点，发现129 I/I比值随深度降低到250×10−15左右，对应于40 Ma的最小年龄，但在几个地点，129 I/I比值的最大值表明年轻碘化物的局部增加。结果表明，大量的碘化物来源于始新世的深源增生沉积物，而晚中新世的碘化物则来自其他源区。孔隙沃茨中老碘化物的存在表明，流体通道是开放的，允许长距离运输到天然气水合物领域。碘化物和甲烷在水合物领域之间的强相关性，再加上在水溶液中的传输参数的相似性表明，在天然气水合物中的甲烷的大部分也有老的来源，并被输送到目前的位置从始新世年龄的源区。
Hydrate Ridge off the coast of Oregon, USA, is a prime example for gas hydrate occurrences in active margin settings. It is part of the Cascadia Margin and was the focus of Ocean Drilling Program (ODP) Leg 204, which successfully recovered fluids from nine sites from the southern part of the ridge. Iodide concentrations in pore fluids associated with gas hydrates are strongly enhanced, by factors up to 5000 compared to seawater, which allows the use of this biophilic element as tracer for organic source regions. We applied the cosmogenic isotope129I (T1/2=15.7 Ma) system to determine the age of the organic source formation responsible for the iodide enrichment. In all sites at ODP Leg 204,129I/I ratios were found to decrease with depth to values around 250×10−15, corresponding to minimum ages of 40 Ma, but in several sites, maxima in the129I/I ratios point to the local addition of young iodide. The results indicate that a large amount of iodide was derived from deep accreted sediments of Eocene age, and that additional source regions provide iodide of Late Miocene age. The presence of old iodide in the pore waters suggests that fluid pathways are open to allow transport over large distances into the gas hydrate fields. The strong correlation between iodide and methane in hydrate fields coupled with the similarity in transport parameters in aqueous solutions suggests that a large fraction of methane in gas hydrates also has old sources and is transported into the present locations from source regions of Eocene age.