Abstract Microbial mats in siliciclastic coastal environments are considered as non-lithifying systems that lack the potential for calcification. This work documents precipitation and preservation of well-defined, laterally continuous carbonate laminae in low-relief microbial mats from a siliciclastic supratidal flat in northern Patagonia (Paso Seco, Argentina). Petrographic, epifluorescence, and SEM-EDS studies of surficial and buried microbial mats show that they are composed of repeated sediment sequences comprised of four different types of laminae, which are, from base to top: (A) a sand and silt lamina, (B) a lamina largely composed of organic matter displaying moulds of subvertical cyanobacteria filaments, (C) a lamina composed of dense micritic carbonate, which is cut by moulds of vertical cyanobacteria filaments, and (D) a lamina composed of organic matter containing abundant horizontal cyanobacteria filaments. The formation of each different lamina is strongly controlled by the environmental conditions, characterized by episodic seawater flooding, followed by several days to weeks in which water remains covering the sediment and gradually evaporates producing a salinity increase and the precipitation of calcite, gypsum and halite. Thus, the basal sand and silt lamina forms as the result of the transport of siliciclastic grains and particles during seawater flooding. The overlying lamina B, composed of organic matter with moulds of subvertical cyanobacteria filaments, is formed when calm water conditions are recovered and organic material is produced by photosynthetic microbial activity. The following lamina C, composed of dense micritic carbonate, precipitates when the salinity of stagnant water reaches CaCO3 supersaturation. Finally, the uppermost lamina D, composed of organic matter with horizontal cyanobacteria filaments, is developed while the sediment surface is drying and gypsum and halite precipitate, although these minerals are not preserved in the sediment because they dissolve during subsequent inundations. All these observations show that well-developed, laterally-continuous carbonate laminae may be formed and preserved in a siliciclastic tidal environment if biotic and abiotic sedimentary processes closely interact to create the required conditions. The studied microbial mats increase sediment impermeability, which favours water retention in the flat and, thus, subsequent evaporation of the retained seawater. Moreover, microbial cells and EPS (extracellular polymeric substance) suspended in seawater might act as nuclei for CaCO3 precipitation, which will later settle down on the microbial mat. In addition, EPS of the microbial mats may also serve as nucleus for in situ carbonate precipitation.

中文翻译：

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在微生物垫定殖的硅质碎屑潮滩中记录的碳酸盐薄层

摘要 硅质碎屑沿海环境中的微生物垫被认为是缺乏钙化潜力的非石化系统。这项工作记录了巴塔哥尼亚北部（阿根廷帕索塞科）的硅质碎屑潮上坪的低地势微生物垫中轮廓分明的横向连续碳酸盐层的沉淀和保存。地表和埋藏微生物垫的岩相学、落射荧光和 SEM-EDS 研究表明，它们由重复的沉积序列组成，由四种不同类型的层组成，从底部到顶部分别是：（A）砂和淤泥层，（ B) 主要由有机物组成的层，显示出亚垂直蓝藻细丝的霉菌，(C) 由致密的泥晶碳酸盐组成的层，由垂直蓝藻细丝的模具切割而成，(D) 由含有丰富水平蓝藻细丝的有机物组成的薄层。每个不同层的形成都受到环境条件的强烈控制，其特征是间歇性海水泛滥，随后数天至数周，水仍然覆盖沉积物并逐渐蒸发，导致盐度增加和方解石、石膏和岩盐的沉淀。因此，基底砂和粉砂层是海水泛滥过程中硅质碎屑颗粒和颗粒输送的结果。当恢复平静的水面条件并且通过光合微生物活动产生有机物质时，就会形成上覆的 B 层，它由带有近垂直蓝藻细丝霉菌的有机物质组成。下面的 C 层由致密的泥晶碳酸盐组成，当死水的盐度达到 CaCO3 过饱和度时沉淀。最后，最上面的 D 层由具有水平蓝藻细丝的有机物组成，在沉积物表面干燥和石膏和岩盐沉淀时发育，尽管这些矿物质没有保留在沉积物中，因为它们在随后的淹没过程中溶解。所有这些观察结果表明，如果生物和非生物沉积过程密切相互作用以创造所需的条件，则可以在硅质碎屑潮汐环境中形成并保存发育良好的横向连续碳酸盐层。所研究的微生物垫增加了沉积物的不渗透性，这有利于平地中的水保持，因此，保留的海水随后蒸发。而且，悬浮在海水中的微生物细胞和 EPS（细胞外聚合物）可能作为 CaCO3 沉淀的核，之后会沉淀在微生物垫上。此外，微生物垫的 EPS 也可以作为原位碳酸盐沉淀的核心。