Here we describe a new reaction-transport model that quantitatively examines δ13C profiles of porewater methane and dissolved inorganic carbon (DIC) (δCCH4 and δCDIC) in the anoxic sediments of the Santa Barbara Basin (California Borderland region). Best-fit solutions of the model to these data suggest that CO2 reduction is the predominant form of methanogenesis in these sediments. These solutions also accurately reproduce the isotope depth profiles, including a broad minimum in the δCDIC profile and a much sharper (angular) minimum in the δCCH4 profile, both of which appear near the base of the transition zone in the sediments between sulfate reduction and methanogenesis (referred to here as the sulfate-methane transition zone, or SMTZ). Such minima in pore-water profiles of δCCH4 near the base of the SMTZ have been seen in a number of other marine sediments across a range of depth and timescales. We show here that this minimum in the δCCH4 profile in Santa Barbara Basin sediments results from the balance between (1) anaerobic oxidation of methane (AOM), which leads to an increase in δCCH4 with decreasing depth in the sediment column through and above the SMTZ; (2) methanogenesis, which produces 13C-depleted methane, both in and below the SMTZ; and (3) an upward flux of CH4 from depth that is relatively enriched in 13C as compared with the methane in these pore waters. Possible sources of this deep methane include the following: geologic hydrocarbon reservoirs derived from ancient source rocks; decomposition of buried gas hydrates; and biogenic (or perhaps thermogenic) methane produced hundreds of meters below the seafloor stimulated by increasing temperatures associated with the sediment geothermal gradient. Although we are unable to resolve these possible sources of deep methane, we believe that the significance of an upward methane flux as an explanation for minima in δCCH4 pore-water profiles may not be limited to Santa Barbara Basin sediments but may be common in many continental margin sediments.

中文翻译：

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用反应传输模型检验的圣巴巴拉盆地（美国）沉积物中的甲烷动力学

在这里，我们描述了一种新的反应传输模型，该模型定量检查了圣巴巴拉盆地（加利福尼亚边境地区）缺氧沉积物中孔隙水甲烷和溶解无机碳 (DIC)（δCCH4 和 δCDIC）的 δ13C 分布。该模型对这些数据的最佳拟合表明，CO2 减少是这些沉积物中产甲烷的主要形式。这些解决方案还准确地再现了同位素深度剖面，包括 δCDIC 剖面中的一个广泛的最小值和 δCCH4 剖面中更尖锐的（角度）最小值，这两者都出现在硫酸盐还原和产甲烷作用之间沉积物过渡带的底部附近（此处称为硫酸盐-甲烷过渡区，或 SMTZ）。在 SMTZ 底部附近的 δCCH4 孔隙水剖面中的这种最小值已经在一系列深度和时间尺度的其他海洋沉积物中看到。我们在此表明​​，圣巴巴拉盆地沉积物中 δCCH4 剖面中的这一最小值源于 (1) 甲烷厌氧氧化 (AOM) 之间的平衡，这导致 δCCH4 随着穿过 SMTZ 及其上方沉积物柱深度的减小而增加; (2) 产甲烷，在 SMTZ 内部和下方产生 13C 耗尽的甲烷；(3) 与这些孔隙水中的甲烷相比，13C 相对富集的深度 CH4 向上通量。这种深层甲烷的可能来源包括：源自古老烃源岩的地质油气藏；埋藏天然气水合物的分解；在与沉积物地温梯度相关的温度升高的刺激下，生物源（或可能是热源）甲烷在海底数百米处产生。虽然我们无法解决这些深层甲烷的可能来源，但我们相信向上甲烷通量作为 δCCH4 孔隙水剖面中最小值的解释的重要性可能不仅限于圣巴巴拉盆地沉积物，而且可能在许多大陆地区很常见。边缘沉积物。