光合作用生物膜建造的凝块:来自于辽东半岛芙蓉统长山组凝块石生物丘中的一些证据

Clots built by photosynthetic biofilms: Evidences from thrombolite bieherms of the Changshan Formation of Cambrian Furongian in Liaodong Peninsula

作为一种以非纹层状组构而区别于叠层石的微生物碳酸盐岩类型,中凝块(mesoclots)是产生凝块石凝块组构关键的微结构。构成这种中凝块的离散性凝块状泥晶,似乎代表了细胞外聚合物质(EPS)复杂的钙化作用,但是,由于常常缺乏直接的微生物证据,造成其所涉及的准确成因和特殊过程还不太清楚。在辽东半岛大连金州湾和本溪田师傅剖面芙蓉统下部的长山组顶部,发育较为典型的凝块石生物丘,组成三级层序的强迫型海退体系域沉积,代表着较为壮观的沉积学现象;在这些凝块石生物丘中,毫米级别大小的暗色离散性凝块状泥晶是其最为基本的沉积组构,而且在这些凝块之中以较高密度保存的丝状蓝细菌鞘化石反映的微生物构成,意味着它们具有明显的光合作用生物膜成因。尽管穿越成岩作用过滤器去解释古代凝块石复杂的形成机理将存在着巨大的挑战,也尽管形成这些凝块石复杂的生物膜钙化作用细节需要更加深入的研究才能得到更好的了解,但是,辽东半岛芙蓉统长山组的凝块石,尤其是在凝块内较为丰富的钙化蓝细菌鞘化石所表征的直接的微生物活动证据,使其成为一个了解光合作用生物膜建造凝块的典型实例;再者,与凝块共生的三叶虫化石、底栖鲕粒和皮壳粒,以及围绕着这些多样化颗粒特别的放射纤维状方解石结壳,说明这些凝块石可能属于较厚的微生物席内多重状生物膜以及构成这些微生物席和微生物膜的细胞外聚合物质复杂的钙化作用建造物。

Thrombolites are marked by non-layer fabrics,which are distinct from stromatolites;the key microstructure of thrombolites are mesoclots that produce clotted fabrics. Diffusely clotted micrite that commonly make up the mesoclots likely represent extracelluar polymeric substances( EPSs) calcification.However,its precise origin and formative processes are not clear due to absence of direct evidence of microbial activities. Thrombolite bioherms occur in the top of the Changshan Formation of the Cambrian Furongian at the Jinzhouwan and Tianshifu sections in the Liaodong Peninsula. Thrombolite bioherms were deposited in the forced regressive system tract of a third-order depositional sequence. Diffusely clotted micrites with the size of micrometers are fundamental fabrics of these thrombolites. Importantly and particularly,the sheath fossils of filamental cyanobateria are preserved,suggesting a biological origin of photosynthetic biofilms. A great challenge is to interpret the ancient clots and their detail formative processes through the filter of diagenesis. Calcification of cyanobacteria biofilms also remains poorly understood. The preservation of cyanobacteria sheath fossils within thrombolite colts in the Furongian Changshan Formation in the Liaodong Peninsula suggests that the colts were formed by photosynthetic biofilms. Furthermore,the clots within thrombolites are commonly symbiosis with other types of carbonate grains such as benthic ooid,cortoid,and trilobite fossil fragment,as well as radiated calcite fabric around these carbonate grains,which indicate that the thrombolites of the Changshan Formation probably resulted from complex calcification of EPSs of multiple biofilms within the relative thick microbial mat.