Did borehole BASE 3 encounter an Archaean, microbially colonized hot springs field in a tidal flat ?

点击翻译按钮获取中文摘要

Tidal-facies sandstones of the Moodies Group in the Barberton Greenstone Belt (~3.22 Ga) were cored by BASE-3 during the 2021-2022 ICDP BASE drilling campaign. They show three unique sedimentary structures: (1) Sandstones are laminated by kerogenous microbial mats; (2) occasionally, calcareous (extensively silicified) microbialites top large fluid-escape structures; (3) innumerable formerly calcareous, tightly laminated, travertine-like micromounds occur in at least one km-long exposed unit drilled by BASE-3; their formation was likely at least microbially mediated. All three structures are confined to the several km-wide thermal halo of the large Lomati River Sill in the central BGB, whose U-Pb zircon crystallization ages are indistinguishable from depositional ages of Moodies Group tuffs. We infer that (1) the Archean shallow-water biosphere was at least in places diverse and abundant; (2) extremely early and surficial carbonate cementation and silicification was responsible for the preservation of this microbially dominated unit; and (3) a regional shallow hydrothermal system degassed, perhaps both diffusely and discretely, into the tidal flat. CO2, CH4, H2O, H2S, and perhaps H2 significantly promoted microbial metabolisms at the surface. We will use thermal modeling, petrographic techniques, outcrop analysis, and high-resolution mapping to compare precise zircon dating, Raman spectroscopy, and mineral paragenesis analysis of outcrop samples with BASE borehole samples from BASE-3. This requires detecting and removing the effects of subsequent regional metamorphism, tectonothermal alteration, and recent oxidative weathering. The objective of this project is to reconstruct depositional facies, thermal conditions, diagenetic processes, and the microbial ecology of this unique depositional and diagenetic environment and to provide a novel perspective on the growth and spreading of life on Early Archaean coastlines. This, in turn, will help to place results from other BASE research projects into proper regional and evolutionary context.