Spatial Distribution and Preservation of Carbon Isotope Biosignatures in Freshwater Microbialite Carbonate

Abstract

Understanding formation mechanisms of modern microbialites enables interpretation of biosignatures associated with fossilized stromatolites. Photosynthetic influences on carbonate precipitation are one proposed mechanism. Photosynthetic isotope biosignatures (13C) associated with freshwater microbialites in Pavilion Lake, British Columbia were widespread through the lake but less prevalent with increasing depth. Importantly, they were variably detectable on the exterior surface of individual microbialites. At depths ≤18 m, microbialite surface carbonates, associated with either nodular microbial communities or surface biofilms, had δ13Ccarb values up to +3.7‰ that were 13C-enriched above the predicted range of δ13Ccarb values for equilibrium precipitation from bulk ambient lake water dissolved inorganic carbon (DIC) (predicted mean δ13Ccarb = −0.2 ± 1.3‰). Vertical profiles of exterior, non-nodular biofilm present on microbialites collected from depths of 21 m and below showed instances of 13C-enrichment near the apex, consistent with hypothesized maximum light exposure. With increased distance from the apex toward the structure base, δ13Ccarb values typically decreased into the predicted range of equilibrium δ13C values. Surface biosignatures persisted internally for distances of 0.5 to 2 cm below the exterior of the structures, beyond which they fell within the predicted isotopic equilibrium range. This shift in δ13Ccarb values may be due to secondary carbonate precipitation masking of the 13C-enriched signature. The contribution of secondary carbonate precipitation was estimated to be 14–59% of total carbonate mass if derived from heterotroph-influenced or bulk lake DIC, respectively. Growth rate estimates suggested these accretion processes can mask photosynthetic signatures in 20–400 years.

Biosignatures Associated with Freshwater Microbialites

“…Similar changes in isotopic composition due to photosynthetic activity have been reported from cultivation studies [19] and from measured differences in δ 13 Ccarb value between material precipitated in association with active cyanobacterial layers (potentially containing microalgals) versus heterotrophic layers of artificial mats [23]. δ 13 Ccarb values that are 13 C-enriched above equilibrium by ~ 2 to 3.5‰ is also consistent with biosignatures identified in Pavilion Lake microbialites [8,24,35]. If the trend seen in the 2012 depth profile of more depleted δ 13 CDIC values at depth was also present when these samples were collected, this isotopic enrichment would be even larger (~5.1‰).…”

“…While previous results have shown that such enrichments are predominantly associated with cyanobacteria-dominated surface nodules [8] and structures at shallow (<11 m) depths [35,36], there have been few observations of The observation of this isotopic biosignature only in the deepest sample was unexpected. While previous results have shown that such enrichments are predominantly associated with cyanobacteria-dominated surface nodules [8] and structures at shallow (<11 m) depths [35,36], there have been few observations of such a biosignature for microbialite surface carbonates at a greater depth in Pavilion Lake, although some have been noted [24]. Further, the lack of isotopic enrichment in the shallower depths in Kelly Lake made this observation all the more unexpected.…”

“…It is worth noting that an isotopic biosignature was not found at 20 m, where the highest concentrations of PLFAs, cyanobacterial-specific 16S copies, and presumably the highest biological activity, were observed. This may be due to differences in the balance of photoautotrophic and heterotrophic influences on the isotopic composition of the local DIC at this depth; a balance that has been suggested to impact the creation and preservation of isotopic biosignatures in Pavilion Lake [24,35]. In particular, Belan and colleagues hypothesized that a shift in the balance of photoautotrophic vs. heterotrophic metabolisms was responsible for the loss of observable 13 C-enriched biosignatures when transitioning from the exterior surface of microbialites to the interior [24].…”

“…This may be due to differences in the balance of photoautotrophic and heterotrophic influences on the isotopic composition of the local DIC at this depth; a balance that has been suggested to impact the creation and preservation of isotopic biosignatures in Pavilion Lake [24,35]. In particular, Belan and colleagues hypothesized that a shift in the balance of photoautotrophic vs. heterotrophic metabolisms was responsible for the loss of observable 13 C-enriched biosignatures when transitioning from the exterior surface of microbialites to the interior [24]. Alternatively, more strongly isolated microenvironments may occur at 26 m, which allows isotopic enrichment of the DIC pool from which the carbonate is being precipitated.…”

Depthwise microbiome and isotopic profiling of a moderately saline microbial mat in a solar saltern

“…Studies have also focused on profiling the isotopic characteristics of these mats to aid in deciphering the functional capabilities of the microorganisms 25 – 28 . Stable isotope profiles can give clues as to the dominant microbial metabolism and mineralogical influences at different layers within the mat 29 . This information, in turn, has important consequences in understanding past environments and climatic conditions 30 – 32 .…”