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

在2021-2022年ICDP BASE钻井期间，对Barberton绿岩带（~3.22 Ga） moody组潮汐相砂岩进行了BASE-3取心。它们具有三种独特的沉积构造：(1)砂岩为干酪根微生物席层状；(2)偶尔，钙质（广泛硅化）微生物岩顶部有大型流体逸出构造；(3)在BASE-3钻探的至少一公里长的暴露单元中，出现了无数以前是钙质的、紧密层合的、石灰华状的微丘；它们的形成至少可能是由微生物介导的。这三个构造都局限于BGB中部Lomati河基底的几公里宽的热晕，其U-Pb锆石结晶年龄与穆迪氏群凝灰岩的沉积年龄难以区分。我们推断：(1)太古宙浅水生物圈至少在某些地方是多样而丰富的；(2)极早的表层碳酸盐胶结和硅化作用是该微生物主导单元保存的主要原因；(3)区域性浅层热液系统向潮滩脱气，可能是扩散的，也可能是离散的。CO2, CH4, H2O, H2S，或许还有H2显著促进了地表微生物的代谢。我们将使用热模拟、岩石学技术、露头分析和高分辨率制图，将露头样品的精确锆石定年、拉曼光谱和矿物共生分析与BASE-3的BASE钻孔样品进行比较。这就需要检测和消除随后的区域变质作用、构造热蚀变和最近的氧化风化作用的影响。该项目的目的是重建这一独特的沉积和成岩环境的沉积相、热条件、成岩过程和微生物生态，并为早期太古代海岸线上生命的生长和传播提供一个新的视角。反过来，这将有助于将其他基础技术基础研究项目的结果置于适当的区域和进化背景中。

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.