High enrichment of molybdenum in hypersaline microbial mats of Guerrero Negro, Baja California Sur, Mexico

- High enrichments of Mo were found in Guerrero Negro hypersaline microbial mats.
- Authigenic biogeochemical processes cause high enrichments of Mo in microbial mats.
- Hypersaline mats could be important sinks of Mo in ancient shallow oceans.
- High Mo enrichments may indicate presence of ancient hypersaline environments.

Analysis of modern microbial mats from the Guerrero Negro (Mexico) hypersaline region showed substantial Mo enrichment factors (EFMo = (Mo/Al)sample/(Mo/Al)background) ranging from 63 to 558, with an average molar ratio value of 297 ± 135. These enrichments are caused by authigenic biogeochemical processes, which are independent of the local abundance of Mo. Our calculations indicate that approximately a half of the Mo enrichment in the microbial mats may be explained by geochemical processes (e.g., coprecipitation with authigenic sulfides such as pyrite), while the remaining half are included in our operationally defined residual fraction. Generally, it is accepted that the elevated concentrations of Mo found in ancient anoxic sediments are a consequence of high biological production. Our calculations of EFMo based on concentrations of Mo and Al taken from the literature for different ancient environments (74 to 155 Ma) suggest that some of these elevated enrichments (EFMo of up to 1261) could have been the result of benthic microbial mats colonizing such environments. Considering that modern microbial mats may be used as proxies to those which have existed on Earth 3.5 billion years ago, the results obtained in this work suggest that these consortia of microorganisms may have represented an important sink for Mo during periods in Earth's history when microbial mats were more abundant. This characteristic leads us to propose high EFMo values, slight Fe impoverishments, low reactive Fe availability, and intermediate Degree of Pyritization (12 to 50%) as a new geochemical tool to indicate conditions associated with the presence of microbial mats in ancient hypersaline environments.