干旱区生态环境知识资源中心

Organic spring deposits have the potential to provide to outstanding records of palaeoenvironmental and climatic change, particularly in arid and semi-arid environments where establishing robust records of environmental change is challenging due to a lack of classic sedimentary records, e.g. perennial lakes and extensive wetlands. However, despite the potential of organic spring deposits a number of studies demonstrate complications in the application of standard C-14 techniques which has, in several cases, led to confusing chronologies. This implies that dynamic carbon pathways commonly occur within spring systems. Because of the importance of springs as critical palaeoenvironmental archives, this study sought to better understand the behaviour of C-14 and other radionuclides used in geochronology within organic springs, and ultimately, establish a protocol for building reliable chronologies in these environments. To do this, we utilised multiple geochronological methodologies to investigate cores collected from three springs in the Kimberley region of northwest Australia. This included C-14 dating of different carbon fractions, Pb-210 dating, the application of Pu239+24, and novel, high spatial resolution, luminescence techniques as indicators of geochronological structure. The natural sensitivity corrected luminescence (L-n/T-n) signal indicated the studied springs contained a relatively complete stratigraphic record, however C-14 results were found to be convoluted by contamination attributed to a combination of roots, groundwater fluctuations and allochthonous input of "old" carbon affecting ages. Whilst it was found that no single carbon fraction is universally reliable in dynamic spring environments, dating the stable polycyclic aromatic carbon (SPAC), isolated by hydrogen pyrolysis (HyPy) pre-treatment, appeared to remove the effects of post-depositional modification which otherwise perturbed the age of carbon fractions with respect to sedimentary development of the spring. By contrast, the ability of Pb-21 and Pu239+24 to provide detailed chronologies for recent spring sediments (i.e. the past 100 years) was found to be complicated due to the behaviour of springs as an open system with regards to uranium. Therefore, it may not be possible to construct Pb-210 chronologies in many spring environments. Overall, the results of this study indicate that it is possible to construct C-14 based chronologies in spring systems, however it is necessary to understand the effects of physical and biological processes within springs on C-14 pathways. In particular, the application of HyPy pre-treatment of SPAC appears to offer a viable approach to constructing chronologies in these environments. Furthermore, although this study pertains to springs, the sources of geochronological complexity described here are not exclusive to these systems and our results are therefore more widely applicable.