A comparative study of low‐temperature carbonate formation on natural exopolymers from hypersaline microbial mats and clay minerals

Abstract

Fluctuations in the abundance of low‐temperature (T) dolomite have occurred throughout geologic time and have been linked to changes in hydrothermal activity, sea level and burial of organic matter suggesting ties to the global biogeochemical cycle of carbon. However, low‐T dolomite remains an enigmatic mineral due to poorly understood controls on its formation. Recent studies have suggested that both biotic and abiotic nucleation surfaces, such as extracellular polymeric substances (EPS) and clays, play an important role in formation. Despite both abiotic and biotic factors having been demonstrated to trigger the formation of low‐T dolomite, comparative studies are lacking. Moreover, the exact mechanisms occurring in the presence of EPS, clays and silica need to be clarified. Thus, this study focused on isolating and characterising EPS from hypersaline sabkhas, a known locale for primary dolomite, as well as clay minerals, and using these materials as reactive surfaces to induce precipitation. Within this study, carbonaceous precipitates formed and were subsequently subjected to high‐resolution analysis including Raman spectroscopy and Near Edge X‐ray Absorption Fine Structure (NEXAFS) to characterise their structure and interaction with specific nucleation sites. The data collected during these experiments has shown that carbonates preferentially nucleate on EPS and most likely on carboxylic functional groups. Additionally, the data show that EPS and clays by themselves may be inadequate to facilitate precipitation. Other factors such as solution chemistry, dissolved Si concentration, as well as organic material and the associated functional groups play a critical role in low‐T dolomite formation. It was also observed that too much organic material could play an inhibitory role. The confirmation of preferential nucleation of carbonates on carboxylic groups lends credence to the currently proposed organomineralisation model of low‐T dolomite precipitation. Furthermore, the data highlight the critical importance of environmental dynamics and their interpretation in the global biogeochemical cycle of carbon.