Cause analysis of travertine blackening:A case study of travertine at Huanglong Ravine,Sichuan Province,China

JIANG Ruiyang; DAI Qunwei; ZHANG Ting; DENG Zhiqing; WANG Lihui; SHAN Jing; RAN Yue; GAO Jun; CAI Jiangrong; WANG Guoyue

doi:10.11932/karst20250512

Abstract：Travertine is one of the most precious resources provided to human beings by nature,and its formation mechanism is highly complex.The formation process involves intricate interactions among multiple factors,including geomorphological features,hydrochemical conditions,hydrodynamic forces,climate variations,environmental changes,and biological activities,all of which collectively preserve critical environmental records.As one of the five world travertine heritage sites,Sichuan Huanglong is famous for its colorful and large-scale travertine landscapes.These unique geological features hold exceptional value for scientific research,tourism,and ecological aesthetics.Every year,Huanglong attracts a large number of tourists from home and abroad,which promotes the rapid development of the local economy.However,in recent years,some of the travertine bodies in the core scenic area of Huanglong have shown obvious blackening phenomena,which not only significantly diminish the aesthetic value of the travertine landscape,but may also indicate the potential changes in the regional ecological environment.At the same time,the blackening of travertine bodies will reduce the quality of the tourists' experience,which will have a certain negative impact on the development of the local tourism industry.As a unique environmental and geological formation developed over tens of thousands of years,travertine landscapes are fragile and non-renewable.Therefore,protecting travertine resources is an urgent matter,and it is important for all of us to work together to preserve them. In order to clarify the causes of travertine blackening,this study focuses on both blackened and unblackened travertine samples from Huanglong,Sichuan Province,China.Representative samples were collected through on-site surveys and analyzed with laboratory testing.A comparative analysis was then conducted between the two types of travertine samples.X-ray diffraction analysis revealed that both the blackened and unblackened travertine corresponded to the PDF#86-2334 standard card,indicating they are calcite-type travertine.X-ray fluorescence spectroscopy results showed that the main chemical composition of both blackened and unblackened travertine were CaO,suggesting no significant changes in the mineral phase composition and chemical components occurred during the blackening process.Further elemental analyses showed that the contents of Total Carbon(TC)and Total Organic Carbon(TOC)of the blackened travertine were significantly higher than those of the unblackened travertine,suggesting that organic matter may play an important role in the blackening process.Meanwhile,Scanning Electron Microscope(SEM)revealed that the biological traces on the surface of the blackened travertine were more pronounced compared to those on the unblackened travertine.Notably,the experiment also detected the presence of substances insoluble in hydrochloric acid on the surface of the blackened travertine.Thermogravimetric analysis of the substance showed that the main mass loss occurred between 200 ℃ and 500 ℃,a range that closely corresponds to the thermal decomposition of organic matter.Therefore,it is inferred that the substance is an acid-insoluble organic matter.These results show that the mineral phase and chemical composition of the blackened travertine remain largely unchanged compared to those of the unblackened travertine,suggesting that the biological effect is the main cause of the travertine blackening. This study provides direct evidence of the biological factors contributing to travertine blackening,offering a new scientific perspective for a deeper understanding of the degradation mechanism of travertine landscapes.It also supplies essential data to support the conservation and management of the Huanglong World Heritage Site.These findings hold great theoretical and practical implications for the conservation and sustainable development of the World Natural Heritage site.