The importance of microbial mats for dolomite formation in the Dohat Faishakh sabkha, Qatar

Abstract

The Dohat Faishakh sabkha in Qatar is one of the rare modern environments where it is possible to study the formation of dolomite, a mineral whose origin has been long debated. In previous studies, dolomite formation in this area was considered to be the result of a penecontemporaneous replacement of aragonite, occurring in the presence of Mg-rich evaporated pore-waters. However, a re-investigation of the sabkha revealed that dolomite is not forming exclusively under the evaporitic conditions that characterize the supratidal zone, but also in microbial mats that colonize the lower intertidal zone, indicating that evaporated pore-waters are not a strict requirement for the mineralization process. Moreover, in the supratidal zone, portions of the sediment that are rich in dolomite are also relatively richer in organic material, which derives from partially degraded microbial mats buried in the sediments. Extracellular polymeric substances (EPS) that constitute microbial mats are recognized as an important component for the formation of Mg-rich carbonates. The presence of living and decaying microbial mats comprising EPS, rather than a replacement process, may be the key factor for dolomite formation in the Dohat Faishakh sabkha.

What do we really know about early diagenesis of non-marine carbonates?

“…In addition, an experimental approach, using in-vitro (Pedley, 2014;Mercedes-Martín et al, 2016) and in-situ (Melim and Spilde, 2011;Boch et al, 2015) setups could help detangle the exact diagenetic pathways (Ritter et al, 2017), controlling parameters and rates of individual processes, across the continuum of non-marine carbonate depositional contexts. A better understanding of the short-lived, intermediate steps and diagenetic products, and the effects on individual geochemical proxies Ritter et al, 2015) is needed to bridge a growing gap between field and lab-based studies at the nanoscale, mostly focussed on the very first steps of carbonate nucleation and precipitation (Benzerara et al, 2006;Sánchez-Román et al, 2011;Krause et al, 2012;Roberts et al, 2013;Burne et al, 2014;Brauchli et al, 2016;Pace et al, 2016), and the rock record of possibly diagenetically modified carbonate deposits, used -or not -in paleoclimatic and -environmental studies.…”

Evidence of a Role for Aerobic Bacteria in High Magnesium Carbonate Formation in the Evaporitic Environment of Dohat Faishakh Sabkha in Qatar

“…Sampling sites were selected based on previous studies confirming the presence of many forms of precipitates and predicting potential dolomite formation (Illing et al, 1965;Brauchli et al, 2016).…”

“…Recent studies in sabkhas in Abu Dhabi and Qatar carried out by Bontognali et al (2010Bontognali et al ( , 2012Bontognali et al ( , 2014b and Brauchli et al (2016), respectively, provided evidence of a close association between dolomite formation and microbial mats, and the authors propose a role for microorganisms in the mineralization process. Dohat Faishakh, a hypersaline coastal sabkha in Qatar, has received much attention since the 1960s as a rare modern geological environment where dolomite formation occurs (Illing et al, 1965).…”

“…Early models of dolomite formation focused on physiochemical processes (Lippmann, 1973;Baker and Kastner, 1981;Lumsden et al, 1995;Brady et al, 1996), while more recent research has emphasized the role of microbial processes in the mineralization process (Sánchez-Román et al, 2008;Sadooni et al, 2010;Bontognali et al, 2012Bontognali et al, , 2014aBrauchli et al, 2016). Earlier models of microbial involvement discussed the role of sulfate reducing bacteria in dolomite formation by increasing pH and alkalinity and removing sulfate (Vasconcelos et al, 1995;Vasconcelos and McKenzie, 1997;Wright, 1999).…”

“…Dohat Faishakh, a hypersaline coastal sabkha in Qatar, has received much attention since the 1960s as a rare modern geological environment where dolomite formation occurs (Illing et al, 1965). The most recent work in Dohat Faishakh, confirmed the importance of living and non-living microbial mats in the formation of authigenic dolomite (Brauchli et al, 2016). Thus, the sabkha represents a unique environment to investigate the involvement of aerobic bacteria in mineral formation.…”