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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >The link between bottom-simulating reflections and methane flux into the gas hydrate stability zone-new evidence from Lima Basin, Peru Margin
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The link between bottom-simulating reflections and methane flux into the gas hydrate stability zone-new evidence from Lima Basin, Peru Margin

机译：底部模拟反射与进入天然气水合物稳定区的甲烷通量之间的联系-来自秘鲁利马盆地的新证据

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Bottom-simulating reflections (BSRs) are probably the most commonly used indicators for gas hydrates in marine sediments. It is now widely accepted that BSRs are primarily caused by free gas beneath gas-hydrate-bearing sediments. However, our insight into BSR formation to data is mostly limited to theoretical studies. Two endmember processes have been suggested to supply free gas for BSR formation: (i) dissociation of gas hydrates and (ii) migration of methane from below. During a recent campaign of the German Research Vessel Sonne off the shore of Peru, we detected BSRs at locations undergoing both tectonic subsidence and non-sedimentation or seafloor erosion. Tectonic subsidence (and additionally perhaps seafloor erosion causes the base of gas hydrate stability to migrate downward with respect to gas-hydrate-bearing sediments. This process rules out dissociation of gas hydrates as a source of free gas for BSRs at these locations. Instead, free gas at BSRs is predicted to be absorbed into the gas hydrate stability zone, BSRs appear to be confined to locations where the subsurface structure suggests focusing of fluid flow. We investigated the seafloor at one of these locations with a TV sled and observed fields of rounded boulders and slab-like rocks, which we interpreted as authigenic carbonates. Authigenic carbonates are precipitations typically found at cold vents with methane expulsion. We retrieved a small carbonate-cemented sediment sample from the seaflood above a BSR about 20 km away. This supported our interpretation the the observed slabs and boulders were carbonates. All these observations suggest that BSRs in Lima Basin are maintained predominantly by gas that is supplied from below, demonstrating that this endmember process for BSR formation exists in nature. Results from Ocean Drilling Program Leg 12 showed that methane for gas hydrate formation on the Peru lower slope and the methane in hydrocarbon gases on the upper slope is mostly of biogenic origin. The δ~(13)C composition of the recovered carbonate cement was consistent with biologic methane production below the seafloor (although possibly above the BSR). We speculate that the gas for BSR formation in Lima Basin also is mainly biogenic methane. This would suggest the biologic productivity beneath the gas hydrate zone in Lima Basin to be relatively high in order to supply enough methane to maintain BSRs.
机译：底部模拟反射（BSR）可能是海洋沉积物中天然气水合物最常用的指标。现在，人们普遍认为BSR主要是由含天然气水合物的沉积物下方的游离气体引起的。但是，我们对BSR形成数据的见解主要限于理论研究。已提出了两种端构件工艺来为BSR的形成提供自由气体：（i）分解气体水合物和（ii）甲烷从下方迁移。在秘鲁海岸附近的德国研究船Sonne的最近一次运动中，我们在经历了构造沉降和非沉积或海底侵蚀的位置发现了BSR。构造沉降（还有可能是海底侵蚀使含天然气水合物的沉积物相对于含天然气水合物的沉积物向下迁移。此过程排除了天然气水合物在这些位置的游离气体作为BSR的自由气源。预计BSRs处的自由气体将被吸收到天然气水合物稳定区中，BSRs似乎局限于地下结构提示流体流动的位置。圆形的巨石和板状岩石，我们将其解释为自生碳酸盐，自生碳酸盐通常是在寒冷的喷口处排出甲烷而形成的沉淀物，我们从BSR上方约20公里处的海驱中回收了一个小的碳酸盐胶结沉积物样本。我们的解释是，观察到的板块和巨石是碳酸盐，所有这些观察结果表明，利马盆地的BSRs是主要的。主要由从下方供应的气体引起，这表明自然界中存在这种形成BSR的末端过程。海洋钻探计划第12条的结果表明，在秘鲁下坡形成天然气水合物的甲烷和上坡形成的烃类气体中的甲烷大部分是生物成因。回收的碳酸盐水泥的δ〜（13）C组成与海床以下（尽管可能在BSR之上）的生物甲烷产量一致。我们推测，利马盆地BSR形成的气体也主要是生物甲烷。这表明利马盆地天然气水合物带下的生物生产力较高，以便提供足够的甲烷来维持BSR。

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