Experimental fossilization of photosynthetic microbial mats in coarse-grained siliciclastic sediments. Experimental fossilization of photosynthetic microbial mats in coarse-grained siliciclastic sediments

Microbial fossils and textures are commonly preserved in Ediacaran and early Cambrian coarse-grained siliciclastic sediments that were deposited in tidal and intertidal marine settings. In contrast, the fossilization of microorganisms in similar marine environments of post-Cambrian age is less frequently reported. Thus, temporal discrepancies in microbial preservation may have resulted from the opening and closing of a unique taphonomic window during the terminal Proterozoic and early Phanerozoic, respectively. Here, we expand upon previous work to identify environmental factors which may have facilitated the preservation of cyanobacteria growing on siliciclastic sand, by experimentally determining the ability of microbial mats to trap small, suspended mineral grains. We show that (1) fine grains coat the sheaths of filamentous cyanobacteria (e.g., Nodosilinea sp.) residing within the mat, after less than one week of cell growth under aerobic conditions, (2) clay minerals do not coat sterile cellulose fibers and rarely coat unsheathed cyanobacterial cells (e.g., Nostoc sp.), (3) stronger disturbances (where culture jars were agitated at 170; 3mm orbital diameter) produce the smoothest and most extensive mineral veneers around cells, compared with those agitated at slower rotational speeds (150 and 0 rpm), and (4) mineral veneers coating cyanobacterial cells are ~1 micrometer thick. These new findings suggest that sheathed filamentous cyanobacteria may be preferentially preserved under conditions of high fluid energy. We integrate these results into a mechanistic model that explains the preservation of microbial fossils and textures in Ediacaran sandstones and siltstones, and in fine-grained siliciclastic deposits that contain exceptionally preserved microbial mats.

微生物化石与微组构通常保存在形成于潮汐及潮间带海洋环境的埃迪卡拉纪和寒武纪早期粗粒硅质碎屑沉积物中。与之相较，寒武纪之后同类海洋环境中的微生物化石则鲜有报道。因此，微生物保存的时间差异可能分别缘于新元古代末期与显生宙早期独特埋藏学窗口（taphonomic window）的开启与关闭。
本文在既往研究基础上，通过实验测定微生物席捕获悬浮细小矿物颗粒的能力，以期识别可能促进硅质碎屑砂表面蓝细菌保存的环境因素。研究结果显示：（1）在有氧条件下细胞生长不足一周后，细粒矿物会附着于栖息于微生物席内的丝状蓝细菌（如诺多丝蓝细菌属未定种，*Nodosilinea* sp.）的鞘层上；（2）黏土矿物不会附着于无菌纤维素纤维，且极少附着于无鞘蓝细菌细胞（如念珠藻属未定种，*Nostoc* sp.）；（3）相较于转速较低（150 rpm及0 rpm）的实验组，更强的扰动条件（培养罐以170 rpm、轨道直径3 mm进行振荡）可在细胞周围形成更为平整且分布更广的矿物覆膜；（4）附着于蓝细菌细胞的矿物覆膜厚度约为1微米。
上述新发现表明，带鞘丝状蓝细菌可能优先在高流体能量条件下得以保存。本研究将上述结果整合至机理模型中，该模型可解释埃迪卡拉纪砂岩、粉砂岩以及产有特异保存微生物席的细粒硅质碎屑沉积岩中微生物化石与微组构的保存机制。