Blooming of a microbial community in an Ediacaran extreme volcanic lake system

Abstract

Ancient aquatic sediments are critical archives for studying early microbial life and the types of environments in which they thrived. The recently characterized Amane Tazgart microbialites in the Anti-Atlas, Morocco, are a rare and well-preserved non-marine deposit that evolved in an alkaline volcanic lake setting during the Ediacaran Period. A multiproxy geochemical toolbox reveals evidence pointing to spatio-temporal ecosystem organization and succession related to changing lake water chemistry. This is marked by secular transition from a cold/dry climate, hypersaline alkaline thermophilic and anoxic-oxic community, to a stable state warm/wet climate fully oxygenated fresh to brackish water ecosystem, predominated by oxygenic stromatolites. Extreme dissolved Arsenic concentrations suggest that these polyextremophiles required robust detoxification mechanisms to circumvent arsenic toxicity and phosphate deficiency. We propose that self-sustaining and versatile anoxic to oxic microbial ecosystems thrived in aquatic continental settings during the Ediacaran Period, when complex life co-evolved with a rise in atmospheric oxygen content.

Figure 1

Rare Earth Element and Yttrium spectra (REE + Y). (a) REY spectra of carbonates. (b) REY spectra of Epiclastic deposits.

Figure 2

(a) Thin section photomicrograph of a thrombolite showing a mesoclot characterized by a micritic core (Ms). (b) RGB image of the distribution of As (red), Fe (green), and Ca (blue) in the area indicated by the yellow square in (a), showing the diffused distribution of As within calcite. (c) Thin section photomicrograph of a stromatolite showing an alternation between iron microbial crusts (Ir) and sparitic crusts (Sp). (d) RGB image of the distribution of As, Fe, and Ca in the area indicated by the yellow square in (c), showing the condensed concentration of As within iron microbial crust. (e) Thin section photomicrograph of a stromatolite composed of the alternation of grain-sized microbial laminae (Gs) and sparitic crusts (Sp), (f) RGB images of the distribution of As, Fe, and Ca of grain-sized microbial laminae and a sparitic crust expressing the enrichment of grain-sized laminae in irons and arsenic.

Figure 3

(a) and (b) As and Ca distribution in a carbonate sample, respectively, in (c). Lighter colors indicate higher XRF intensities (higher elemental concentrations). The typical crystal shape of the nano-/micro-carbonates reflected in the carbonate distribution indicates that As is distributed homogenously within these phases. (c) RGB image of the distribution of As, Fe and Ca showing the points where XANES spectra were measured. (d) XANES spectra at the As k-edge were obtained in three representative points from (c).

Figure 4

(a) RGB of As, Fe, and Ca distribution in a carbonate sample showing the association of As within iron, with an inset view of the disseminated distribution of As marked by a white square for (b), corresponding to point 1 of the XANES spectra. (c) RGB image of As, Fe, and Ca distribution measured in a different location of the same sample as (a), showing two points where XANES spectra were performed. (d) XANES spectra at the As K-edge were obtained in three representative points from both (a) and (c).

Figure 5

(a) A-CN-K ternary diagram with uncorrected data. (b) A-CN-K ternary diagram with corrected data. CIA–Chemical Index of Alteration, CIAcorr–Corrected CIA, A–Al₂O₃, CN–CaO* + Na₂O, K–K₂O (all in molar proportions), CaO*–CaO incorporated in the silicate fraction of the sample. Note: average Upper Continental Crust (UCC; Taylor and McLennan); Post-Archean average Australian Shale (PAAS: Taylor and McLennan); Andesite; Granite; TTG and Basalt (Condie).

Figure 6

Stratigraphic profile of enrichment factors (EF) of selected redox trace elements, the chemical index of alteration (CIA), and the Y/Ho ratios as a salinity proxy of Amane Tazgart succession. (EF of Co, Cr, Ni, Th, U, V, and Zn, were calculated relative to the average shale PAAS (Taylor and McLennan), and As relative to the UCC (McLennan). The lithostratigraphic column is modified from Chraiki et al..

Figure 7

Salinity proxies for Amane Tazgart samples. (a) Biplots of B versus Ga. (b) B versus K (%).

Figure 8

(A) Biplots of Ce/Ce* eq2 and Al showing a moderate positive correlation for carbonates and no correlation for epiclastic samples. (B) Amane Tazgart carbonates and epiclastic sediments in a graph of Ce/Ce* vs Pr/Pr* (Bau and Dulski).

Figure 9

Detailed lithostratigraphic column associated with thin section pictures of the different analyzed samples of the Amane Tazgart succession (the lithostratigraphic column is modified from chraiki et al.). Yellow arrows mark the analyzed parts of the samples (1 corresponds to primary fabrics, 2 correspond to early diagenetic phases).