Organic influences on hydrated magnesium carbonate mineral formation

Abstract

Carbonate minerals retain organic compounds and preserve textural and chemical evidence of microbial activity early in the geologic record of Earth. For this reason, magnesium carbonates thought to be associated with lacustrine deposits in Jezero Crater are an important target of the Mars Sample Return Mission. The presence of hydrated magnesium carbonates in the deposits suggests that these minerals experienced minimal postdepositional alteration and may have the potential to preserve biosignatures from a habitable early martian environment. Microbial influences on calcium carbonate precipitation are well documented, but magnesium carbonates have received considerably less attention as a result of their relative scarcity in terrestrial deposits. The few modern lacustrine environments where hydrated magnesium carbonate minerals form have been proposed as analogs for Jezero Crater. Precipitation often occurs in association with microbial communities in these alkaline lake systems, but little is known about the potential for hydrated magnesium carbonates to preserve biosignatures, especially in depositional environments analogous to the carbonate sediments and coatings identified by the Perseverance Rover in Jezero Crater. This thesis explores organic influences on hydrated magnesium carbonate precipitation and the potential for these minerals to retain evidence of microbial activity. I begin by culturing cyanobacterial biofilms in solutions that replicate natural lacustrine environments where hydrated magnesium carbonate precipitation occurs. I designed experiments to isolate the role of cyanobacterial extracellular polymeric substances (EPS) in mediating the mineralogy of hydrated magnesium carbonate precipitates and rate of amorphous magnesium carbonate (AMC) maturation. I also compared the precipitates that formed in association with cyanobacterial biofilms to those formed under inorganic conditions to determine if hydrated magnesium carbonates preserve biosignatures. The results from these experiments demonstrate that cyanobacterial EPS promotes the early stabilization of the hydrated magnesium carbonate mineral dypingite and that biologically associated precipitates encapsulate cells and retain organic compounds detectable with Raman spectroscopy. I complement this laboratory work by seeking to identify similar spectroscopic and textural evidence of microbial activity in a range of carbonate deposits from Lake Salda, Türkiye including sands, crusts on coarse siliciclastic sediments, and alteration veins in serpentinized ultramafic bedrock. Analyses of these samples revealed that hydromagnesite sands and crusts have a higher potential to preserve biosignatures than dolomite veins in a system analogous to Jezero Crater.