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

在2021-2022年ICDP基地钻探活动中，Barberton绿岩带Moody群潮相砂岩(~3.22Ga)被碱基3取心。它们显示出三种独特的沉积结构：(1)砂岩由干酪生微生物席分层；(2)偶尔，钙质(广泛硅化)的微生物岩位于大型流体逃逸结构的顶部；(3)在Base-3钻探的至少一个长达1公里的裸露单元中，至少出现无数以前钙质的、紧密分层的、类似钙华的微丘；它们的形成可能至少是微生物作用的。这三个构造都局限于BGB中部大型Lomati River Sill的几公里宽的热晕，其锆石U-Pb结晶年龄与Moody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存在多样性和丰度的地方；(2)极早期的表层碳酸盐胶结作用和硅化作用是这个微生物控制的单元得以保存的原因；(3)区域浅层热液系统可能扩散和离散地脱气进入潮滩。CO_2、CH_4、H_2O、H_2O、H_2S，可能还有H_2，显著促进了表层微生物的代谢。我们将使用热模拟、岩石学技术、露头分析和高分辨率填图来比较露头样品和基地-3基孔样品的精确锆石测年、拉曼光谱和矿物共生分析。这需要检测和消除后续区域变质作用、构造热蚀变和最近氧化风化的影响。该项目的目标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，并为太古宙早期海岸线上生命的生长和扩散提供一个新的视角。反过来，这将有助于将其他基础研究项目的结果放入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.