Barberton Archean Surface Environments, Moodies Group, Barberton Greenstone Belt
Africa, South Africa, Moodies, Barberton
Revised Full-proposal: ICDP-2019/03
For the funding-period starting 2019-01-15
For the funding-period starting 2019-01-15
by Christoph Heubeck, Nicolas J. Beukes (deceased), Martin Homann, Emmanuelle J. Javaux, Takeshi Kakegawa, Martin J. van Kranendonk, Stefan V. Lalonde, Paul R.D. Mason, Michael Tice
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. In order 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, we propose to drill five key sections. Nine inclined drillholes, each of 350-600 m length (MD), will provide maximum information across a range of terrestrial-marine facies transitions. Our proposal seeks funding for drilling and related field operations in the austral winter seasons of 2020. It documents the global significance and scientific rationale, describes the selected sections, suggests testable hypotheses, and provides the technical details related to drilling, logging, core processing, management, and environmental aspects. The “earth system” and “global environmental change” themes of this proposal provide an excellent match to the themes of the recently announced UNESCO World Heritage Site encompassing these strata.
- We will investigate (1) conformable terrestrial-marine transitions for environmental proxies;
- (2) diagnostic lithologies (such as various paleosols, nearshore BIFs, evaporites and basaltic lavas) for their environmental significance;
- (3) the compositional, facies and morphological variability of thick and laterally extensive microbial mats; and (4) sedimentary and mineralogical responses to surface variables, such as tides, climate, potential meteorite impacts, and radiation, in particular in deep-water strata. Principal questions include:
- (1) What was the ecology, 3-D morphology, and metabolism(s) of abundant (oxygenic photosynthetic?) microbial mats preserved in minimally compacted tidal-facies sandstones? Are these properties recorded in their C-isotope microstratigraphy? What were the preservation pathways, origins of early diagenetic chert, and degrees of thermal overprint? Can we constrain net O2 production rates and the early N cycle?
- (2) What is the (cyclo-)stratigraphic and microfossil record of fine-grained marine and prodelta sediments ? What is the origin of its clay minerals? How do coastal BIFs and jaspilites relate to nearby tidal microbial mats? What does the magnetostratigraphic record imply about the strength of the Paleoarchean magnetic field?
- (3) What global surface conditions can be inferred? What was the redox state (sulfate, redox-sensitive metal isotopes), temperature and composition of ocean water, of early diagenetic fluids, and of the atmosphere?
- (4) What can we infer about the role and significance of terrestrial weathering from proxies of physical and geochemical weathering, the composition of variable paleosols, the architecture of eolian strata and the traces of evaporites and microbial metabolism(s) in terrestrial sediment?
Africa, Barberton, Early Life Ecology, Greenstone Belt, Moodies, Ocean And Atmosphere, Oxygen, South Africa