Patterns of metal distribution in hypersaline microbialites during early diagenesis: Implications for the fossil record

Abstract

The use of metals as biosignatures in the fossil stromatolite record requires understanding of the processes controlling the initial metal(loid) incorporation and diagenetic preservation in living microbialites. Here, we report the distribution of metals and the organic fraction within the lithifying microbialite of the hypersaline Big Pond Lake (Bahamas). Using synchrotron-based X-ray microfluorescence, confocal, and biphoton microscopies at different scales (cm-μm) in combination with traditional geochemical analyses, we show that the initial cation sorption at the surface of an active microbialite is governed by passive binding to the organic matrix, resulting in a homogeneous metal distribution. During early diagenesis, the metabolic activity in deeper microbialite layers slows down and the distribution of the metals becomes progressively heterogeneous, resulting from remobilization and concentration as metal(loid)-enriched sulfides, which are aligned with the lamination of the microbialite. In addition, we were able to identify globules containing significant Mn, Cu, Zn, and As enrichments potentially produced through microbial activity. The similarity of the metal(loid) distributions observed in the Big Pond microbialite to those observed in the Archean stromatolites of Tumbiana provides the foundation for a conceptual model of the evolution of the metal distribution through initial growth, early diagenesis, and fossilization of a microbialite, with a potential application to the fossil record.

Formation of stromatolite lamina at the interface of oxygenic–anoxygenic photosynthesis

“…The deepest anoxic layers of mats typically contain sulfides and are dark colored (gray to black) from iron sulfide precipitation (Des Marais, 2003;Sforna et al, 2016).…”

“…This is of critical importance when interpreting the biogeochemical conditions during formation of fossil microbialites. The minerals comprising these fossils are typically the transformation product of primary (microbial) phases, indicating the need to study diagenetic pathways in extant microbialites (Pace et al., ; Sforna et al., ).…”

“…A green layer of cyanobacteria at the subsurface uses short wavelength (the blue‐green part of the spectrum), and underneath the green layer, purple sulfur bacteria form a red lamina where they harvest photons with a longer wavelength that penetrate deeper into the sediments. The deepest anoxic layers of mats typically contain sulfides and are dark colored (gray to black) from iron sulfide precipitation (Des Marais, ; Sforna et al., ).…”

“…As a secondary effect, a high cation binding capacity inhibits carbonate precipitation, as was observed in the green layer of CCLN mats. The degree of cation binding depends on the specific EPS properties, which change with depth in microbial mats (this study, Braissant et al, 2009;Sforna et al, 2016).…”

“…The combined metabolic reactions of the microbial community alter the geochemical conditions in the mat and result in extreme diel fluctuations of, for example, oxygen, or sulfide concentrations and pH (Dupraz et al., ). The high rates of carbon cycling are a consequence of tight coupling of CO 2 fixation notably through oxygenic and anoxygenic photosynthesis (using either H 2 O or reduced S, H 2 , Fe 2+ , or As 3+ as electron donor, respectively) and aerobic and anaerobic organic carbon oxidation (using, e.g., SO 4 2− , Fe 3+ , or NO 3 − as oxidant; Sforna et al., ; Visscher & Stolz, ). The individual metabolic reactions each have a different effect on the geochemical conditions in the mat.…”