The primary input of organic matter to almost all marine sediments comes from deposition at the sediment surface. However, in many continental margin settings, reduced carbon can also be added to sediments from below—for example, from “deep” geologic hydrocarbon reservoirs derived from ancient source rocks or from the decomposition of deeply buried gas hydrate deposits. To examine the impact of these two differing reduced carbon inputs on sediment biogeochemistry, a modified reaction-transport model for anoxic marine sediments is described here and applied to data from sediment cores in Santa Barbara Basin to a depth of 4.6 m. Excellent model fits yield results consistent with previous studies of Santa Barbara Basin and other continental margin sediments. These results indicate that authigenic carbonate precipitation in these sediments is not centered around the sulfatemethane transition zone (SMTZ), as is seen in many other sedimentary environments but occurs at shallower depths in the sediments and over a relatively broad depth range. Sulfate profiles are linear between the surface sediments (upper ∼20 cm) and the top of the SMTZ (∼105 cm) giving the appearance of refractory particulate organic carbon (POC) burial and conservative sulfate behavior in this intermediate region. However, model results show that linear profiles may also occur when high rates of sulfate reduction occur near the sediment surface (as organoclastic sulfate reduction [oSR]) and in the SMTZ (largely as anaerobic oxidation of methane) with low, but nonzero, rates of oSR inbetween. At the same time, linearity in the sulfate profile may also be related to downward pore-water advection by compaction and sedimentation plus a decrease with depth in sulfate diffusivity because of decreasing porosity. These model-determined rates of oSR and methanogenesis also result in a rate of POC loss that declines near-continuously in a logarithmic fashion over the entire sediment column studied. The results presented further here indicate the importance of a deep methane flux from below on sediment biogeochemistry in the shallower sediments, although the exact source of this methane flux is difficult to ascertain with the existing data.

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圣巴巴拉盆地沉积物中的碳循环：模拟研究

几乎所有海洋沉积物中有机质的主要输入来自沉积物表面的沉积。然而，在许多大陆边缘环境中，还原碳也可以从下方添加到沉积物中——例如，来自古老烃源岩的“深层”地质油气藏或来自深埋天然气水合物沉积物的分解。为了检查这两种不同的碳输入减少对沉积物生物地球化学的影响，这里描述了一种用于缺氧海洋沉积物的改良反应传输模型，并将其应用于来自圣巴巴拉盆地沉积物核心的数据，深度为 4.6 m。出色的模型拟合产生的结果与之前对圣巴巴拉盆地和其他大陆边缘沉积物的研究一致。这些结果表明，这些沉积物中的自生碳酸盐沉淀并不像在许多其他沉积环境中所见的那样以硫酸盐甲烷过渡带 (SMTZ) 为中心，而是发生在沉积物中较浅的深度和相对较宽的深度范围内。表层沉积物（上部~20 厘米）和 SMTZ 顶部（~105 厘米）之间的硫酸盐剖面是线性的，在这个中间区域出现了耐火颗粒有机碳 (POC) 掩埋和保守的硫酸盐行为。然而，模型结果表明，当沉积物表面附近（如有机碎屑硫酸盐还原 [oSR]）和 SMTZ（主要是甲烷的厌氧氧化）以低但非零速率发生高速率硫酸盐还原时，也可能出现线性剖面oSR 介于两者之间。同时，硫酸盐剖面的线性也可能与压实和沉积导致的向下孔隙水平流以及由于孔隙度降低而导致的硫酸盐扩散率随深度的降低有关。这些模型确定的 oSR 和产甲烷率也导致 POC 损失率在整个研究的沉积柱中以对数方式近乎连续下降。这里进一步展示的结果表明，来自下方的深层甲烷通量对浅层沉积物中沉积物生物地球化学的重要性，尽管这种甲烷通量的确切来源很难用现有数据确定。这些模型确定的 oSR 和产甲烷率也导致 POC 损失率在整个研究的沉积柱中以对数方式近乎连续下降。这里进一步展示的结果表明，来自下方的深层甲烷通量对浅层沉积物中沉积物生物地球化学的重要性，尽管这种甲烷通量的确切来源很难用现有数据确定。这些模型确定的 oSR 和产甲烷率也导致 POC 损失率在整个研究的沉积柱中以对数方式近乎连续下降。这里进一步展示的结果表明，来自下方的深层甲烷通量对浅层沉积物中沉积物生物地球化学的重要性，尽管这种甲烷通量的确切来源很难用现有数据确定。