The fate of Arsenic during early microbialite taphonomy: Implications for chemical biosignature preservation

Organo-sedimentary structures built by benthic microbial communities, known as microbialites, dominated the fossil record for the first 3 billion years of Earth’s history. Various microbial metabolisms contribute to microbialite lithification, each of which can be based on biogeochemical cycling of elements capable of supporting life. Arsenic (As), a common element on the surface of Precambrian Earth, has been proposed to have supported the development of early life associated with the construction of primitive microbial carbonates. These As-based metabolisms have left evidence of their existence within the 2.7 Ga old Tumbiana stromatolites, showing the potential of this metalloid to serve as an archive of the dynamic interplay between microbes, minerals, and their environment of deposition throughout Earth’s history. However, significant changes in the geochemical composition of microbialites likely occur during early taphonomic modification and later diagenetic alteration. Therefore, establishing the mechanisms driving the arsenic geochemistry of ancient microbialites can be challenging.

Motivated by these challenges, our objective was to evaluate the mechanisms controlling the initial incorporation of arsenic into actively accreting microbialites, as well as the preservation of the [As] signal during early taphonomic alteration of the structure. Hamelin Pool (Western Australia) is one of the few modern systems that host As-based metabolisms in the microbial communities involved in microbialite accretion. Conventional terminology recognizes four types of microbial mats that produce recognizable internal microfabrics in Hamelin Pool microbialites: pustular, smooth, colloform, and transitional mat types. Over time, these initial microfabrics all follow a similar evolution subdivided into two successive stages: (1) precipitation of micrite along laminations and around clots and; (2) precipitation of aragonitic marine cement. Therefore, Hamelin Pool microbialite fabrics provide a unique and step-wise window into the processes that form ancient microfabrics, particularly highlighting the importance of their early taphonomic evolution in the fate of the As biosignal originally incorporated during initial accretion of the structure.

Based on microbialites collected from Hamelin Pool that have been characterized petrographically, we evaluated the evolution of [As] recorded in the Hamelin Pool microbialites at all stages of deposition and early taphonomic modifications. Results were interpreted in relation to the distinct microbial mats and their metabolisms, as well as the physicochemical and geological variability of the depositional environment. To accomplish this, we conducted a sequential leaching experiment to chemically isolate the organic matter and carbonate fractions, and measured As concentrations on a triple-quadrupole inductively coupled mass spectrometer (Agilent 8900 ICP-QQQ). Preliminary results show that elevated As concentrations are initially incorporated into microbial organic matter before being transferred to the carbonate fraction through successive stages of early taphonomic alteration. Because the carbonate fraction is diagenetically more resistant than the organic matter, this discovery could have major implications for the preservation of geochemical biosignatures in the geological record of microbialites. Our results serve as a first step towards improving the utility of [As] as an indicator of biogenicity in the fossil record of early Earth and, possibly, other planets such as Mars.