基于微岩相分析的藻类在钙华沉积中的作用研究——以四川黄龙为例

在地表环境下，钙华沉积常常是物理化学和生物沉积过程共同作用的结果。藻类因其在钙华沉积环境中具有较大的生物量及其自身拥有多样的代谢方式，对钙华沉积过程和形态具有重要影响。本研究以四川黄龙钙华为例，通过对典型沉积点的水化学、藻类群落组成和现代钙华微岩相结构进行综合分析，来揭示藻类在钙华沉积中的作用。研究发现，黄龙钙华沉积环境中分布的藻类主要包括蓝藻、绿藻和硅藻等。这些藻类代谢活动会在一定程度上改变沉积水体水化学环境，但在快速流动的水体中，其影响有限。不同藻类群落常常形成几百微米至1~2 mm厚的微生物席或生物膜层，作为碳酸钙沉积发生的重要场所，即钙华沉积活动层。在该活动层内，藻体及其分泌的胞外聚合物（EPS）能够为碳酸钙晶体生长提供大量成核位点和生长模板，从而极大地促进钙华沉积。同时，EPS可以控制或影响碳酸钙结晶形态及钙华微岩相结构。准确认识和量化藻类在钙华沉积中的作用还需要继续开展更多微观尺度方面的研究，以便更好地理解钙华沉积机制，并为准确解译古老钙华岩相结构和地球化学特征奠定基础，同时为预测钙华景观演化和保育提供更多科学依据。 

Role of algae in travertine deposition revealed by microscale observations: A case study of Huanglong, Sichuan, China

On the Earth’s surface with an open-water setting，travertine deposits generally result from the interplay of physio‐chemical（abiotic）and biotic processes responsible for carbonate precipitation. Algae is widespread in travertine depositing environments，and the growth of which has an important influence on the process of travertine deposition and travertine macro-and micro-morphology. In this work，we conducted a systematic investigation into hydrochemical compositions of deposition sites，the composition of living algae community and the micro-structure and fabrics of neo-precipitated travertine in the Huanglong area，Sichuan Province. Our objective was to examine the role of algae during the process of travertine deposition. A large number of prokaryotic and eukaryotic algae，such as cyanobacteria，green algae and diatoms，were found to grow in the water environment of Huanglong. The metabolism of them（both photo-synthesis and respiration）could lead to changes in the chemical compositions of water in which travertine forms，but this is only observable in pools with stagnant water. Mostly，algae forms microbial mats or biofilms which are several hundred microns to 1-2 mm in thickness and serve as an important place for carbonate precipitation（i.e. active depositional zones）. Within such zones，the algae and secreted extracellular polymer substances（EPS）likely provide many favorable nucleation sites and growth templates for carbonate crystals，thereby greatly promoting travertine precipitation. Meanwhile，the EPS could control or affect the morphology of calcite crystals and travertine fabrics. Better understanding and quantification of the role of algae in travertine deposition require further study at a microscale，so as to gain more insights into the mechanism of travertine deposition，lay a foundations for accurate interpretation of fabrics and geochemical proxies of ancient travertine deposits and provide more evidence for prediction of the evolution and conservation of travertine landscape.