Possibilities and problems associated with travertines and tufas in Quaternary studies: A case of the Tibetan Plateau

Abstract

Travertines and tufas, precipitated from waters (hydrothermal and ambient temperature karstic waters respectively) enriched in calcium and bicarbonate ions, are widespread in continental settings. Both carbonate deposits are important archives for Quaternary studies, as they can be absolutely dated (e.g. via uranium-series dating) and provide evidence of climatic and environmental conditions at the time of formation. Yet, most of the travertines and tufas in China still lack investigation and little is known about the nature of geological and paleoenvironmental information stored in these deposits. Here we summarize previous studies of travertines and tufas from the Tibetan Plateau, with an emphasis on possibilities and problems associated with these carbonates in Quaternary studies, and lastly provide our perspectives on the future of travertine and tufa research. Travertines are quite common on the Tibetan Plateau and preferentially occur along active tectonic zones, such as the approximately north-south trending rift systems and some strike-slip faults caused by the uplift of the Plateau and subsequent east-west extensional deformation of the Tibetan crust during the Late Cenozoic. At present these travertines are under-researched, but several previous studies have shown that the travertine deposition on the Plateau is not only controlled by tectonics but also closely associated with the intensity of the Indian summer monsoon. Some lacustrine tufas deposited on the paleoshorelines of several closed-basin lakes on the Plateau are able to reconstruct lake-level (and hence paleomonsoon) change. It is therefore expected that the widely-distributed travertines and tufas on the Tibetan Plateau have great potential to provide insight into past monsoonal climate change. Furthermore, travertines and tufas preserved in different terrestrial settings, such as periglacial slope, fluvial, and lacustrine environments on the Plateau, appear to be a promising tool for unravelling climate-driven surface processes and geomorphological change of an area. The formation of travertines (e.g. travertine fissure-ridge) is often linked with active tectonics and can thus provide more evidence of faulting and geothermal activities on the Plateau. In several instances, archeological finds (e.g. ancient human hand- and footprints) are stratigraphically or spatially associated with travertines or tufas; thus the dating of these carbonates may provide the timing of human migration and settlement of the Tibetan Plateau, and allow us to better understand the prevailing climatic and environmental conditions during site occupation and abandonment. Several problems might be encountered in the study of the travertines and tufas. Firstly, the travertines and tufas tend to accumulate discontinuously due to the tempo-spatial shift in spring orifice location and water flow pattern, and may be mixed with terrigenous sediments. Another problem is that a variety of travertine and tufa lithofacies and fabrics within these deposits are observed, and thus a detailed petrographic analysis is needed. In addition, these carbonate deposits are prone to early diagenetic alteration as they commonly have relatively high initial porosity and permeability, resulting in modification of the original geochemical composition, and hence, the paleoclimatic proxy signals recorded in these deposits. Thus, careful analysis must be conducted in order to distinguish primary fabrics from altered material. Last but not least, the processes responsible for travertine and tufa precipitation are extremely complex, and often lead to difficulties in the interpretation of climatic and environmental signals stored in these deposits. Further research should focus on determining the relationships between travertine and tufa proxies (e.g. oxygen and carbon stable isotopes) and climate (temperature, precipitation) based on field measurements and experiments carried out in different depositional environments, such as proximal vents, intermediate slopes, or distal flats and depressions. Considering the problems mentioned above, we suggest that a multidisciplinary approach, including geomorphological, sedimentological, petrographic, geochronological and geochemical investigations, is necessary in the study of travertines and tufas.