The shallowest sediment of three mud volcanoes (MVs) located in the middle slope of the Gulf of Cádiz, Anastasya, Pipoca and St. Petersburg, have been seasonally studied during June and December 2016. These structures are locally important contributors of many biogeochemical active substances to the water column, with a special attention in the emission of methane (CH₄). Along this study, the role of organic matter diagenesis and its contribution to the diffusive fluxes estimated in the sediment–water interface from the three MVs have been investigated mainly. For this, the first combined analyses of sediment properties (granulometry, porosity, density, organic carbon) and pore water chemistry (major elements, dissolved inorganic carbon (DIC), nutrients, CH₄ and nitrous oxide) have been carried out. Anastasya and St. Petersburg MVs presented similar behaviours, with a slight concentration variation for the chemical variables in the top layers of the sedimentary record followed by a more intense vertical variation in deeper layers. Anastasya MV showed some typical processes of these structures, such as the clay dehydration, the dissolution of halite and the precipitation of authigenic carbonates, which was also observed in St. Petersburg MV. Organic matter diagenesis clearly altered the biogeochemical profiles, except for Pipoca MV. Moreover, mud breccia with a mousse-like texture has been identified in the deepest levels of Anastasya and St. Petersburg MVs. Here, anaerobic oxidation of methane is linked to the decrease of \({\mathrm{SO}}_{4}^{2-}\) and the increases of CH₄ and DIC with the core depth. Changes across the sediment–water interface presented low diffusive fluxes in the three MVs due to the presence of other processes as the irrigation by benthic macrofauna, the action of bottom currents and/or the upward fluid migration. From the obtained results, we can conclude that Anastasya MV presents a certain venting activity at present.

Graphical abstract

中文翻译：

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加的斯湾泥火山表层沉积物的生物地球化学

2016 年 6 月和 12 月期间，对位于加的斯湾中坡、阿纳斯塔西亚、皮波卡和圣彼得堡的三座泥火山 (MV) 的最浅沉积物进行了季节性研究。这些结构是许多生物地球化学活性物质的局部重要贡献者到水柱，特别注意甲烷 (CH ₄ )的排放。在这项研究中，主要研究了有机质成岩作用的作用及其对三个 MV 沉积物 - 水界面中估计的扩散通量的贡献。为此，首次对沉积物特性（粒度、孔隙度、密度、有机碳）和孔隙水化学（主要元素、溶解无机碳 (DIC)、养分、CH ₄和一氧化二氮）。Anastasya 和圣彼得堡 MV 表现出类似的行为，沉积记录顶层的化学变量浓度略有变化，随后更深层的垂直变化更为强烈。Anastasya MV 显示了这些结构的一些典型过程，如粘土脱水、岩盐溶解和自生碳酸盐的沉淀，这在圣彼得堡 MV 中也观察到。除 Pipoca MV 外，有机质成岩作用明显改变了生物地球化学特征。此外，在 Anastasya 和 St. Petersburg MVs 的最深处发现了具有类似慕斯质地的泥角砾岩。在这里，甲烷的厌氧氧化与\({\mathrm{SO}}_{4}^{2-}\)CH ₄和 DIC 随岩心深度的增加而增加。由于其他过程的存在，如底栖大型动物的灌溉、底流的作用和/或向上的流体迁移，沉积物 - 水界面的变化在三个 MV 中呈现出低扩散通量。从获得的结果可以得出结论，Anastasya MV 目前呈现出一定的排气活动。

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