Beschreibung
The evolutionary development of oxygenic photosynthesis is a key question in early Earth research because it was and is responsible for the profound transformation of surface environments across our planet and allowed the rise of eukaryotic and complex multicellular life. Various geochemical clues suggest that there were at least temporary variations in the overall very low level of atmospheric oxygen by ~3 Ga. This is consistent with results of recent molecular clock analyses that suggest the onset of oxygenic photosynthesis prior to that time, probably via microbial consortia which included highly productive benthic cyanobacteria that colonized early shorelines. The oldest strata suitable to test the hypothesis of – perhaps local and/or temporary – oxygenation are the ca. 3.2 billion-year-old sedimentary (and minor volcanic) units of the Moodies Group in the Barberton Greenstone Belt, South Africa. These record surface processes in very well preserved and correlatable fluvial-to-prodeltaic siliciclastic rocks; in addition, the ~3.7 km thick strata provide an extremely high resolution (mean ~1 km/Ma) over a relatively short interval of 1-14 Ma. Despite tight regional folding, the metamorphic grade is only lower greenschist facies, and there is a nearly complete absence of penetrative strain due to widespread early-diagenetic silicification. This has preserved abundant primary micro- and macrotextures. Mapping has documented paleosols, terrestrial evaporites, potentially eolian strata, shoreline systems, tidal microbial mats, deltaic complexes, and marine ferruginous sediments / BIF. They provide a worldwide unique opportunity to robustly reconstruct early bio-geo-atmo-hydrosphere processes and conditions, particularly those related to diverse and well-documented microbial life, at an unrivalled level of regional and temporal resolution and during a critical period in Earth history. The drilling program had the overall objective to drill unweathered strata to avoid the effects of oxidative weathering, a particular problem in fine-grained strata, and to obtain continuous sections suitable for geochemical and time-series analyses.
From November, 2021, through July, 2022, we drilled eight inclined boreholes between 280 and 495 m length by drilling through steeply to subvertically dipping, in part overturned strata. Sections and locations had been prioritized during a 2017 field workshop to (1) provide information that could not be obtained from surface outcrops; (2) address specific scientific questions relating to surface conditions of the Archaean Earth; (3) build on significant prior field studies and laboratory work; and (4) allow technically feasible drilling at economically reasonable costs. In addition to the drilling program, three mine adits and tunnels exposing ca. 1000 m of Moodies Group section were also sampled. All boreholes were logged. Up to three rigs operated concurrently, delivering daily ten to sixty m of high-quality core which was slabbed, described and photographed, typically on the following day, in a large, publicly accessible hall adjacent to the museum in downtown Barberton. An exhibition provided background explanations for visitors and related this project to the recently established Barberton-Makhonjwa Mountains World Heritage Site in which most boreholes were located. A substantial Education, Outreach and Publicity program accompanied the drilling campaign. The archive half of the core is stored at South Africa's National Core Repository in Donkershoek near Pretoria; the working half of the core was shipped to the ICDP core repository at the BGR-Berlin facilities in Berlin Spandau. As of January 2023, initial overview geochemical sampling and scanning is ongoing in preparation for a sampling workshop by the BASE science team.