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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - ELEMIN (How the earliest life on Earth became mineralized)

Teaser

Summary

- What is the problem/issue being addressed?
Project ELEMIN targets geochemical and petrographic investigations of Proterozoic and Archean stromatolites, i.e. lithified microbial mats, to (I) determine the genesis of mineral phases preserved within the stromatolites and (II) reconstruct physico-chemical environmental conditions prevailing in microbial habitats. The timing and conditions of formation of individual mineral phases in stromatolites is under debate and, thus, their reliability as geochemical archives is uncertain. One controversy is whether different mineral phases record the elemental composition derived from continental weathering or from submarine hydrothermal sources.

- Why is it important for society?
The evolution and environmental conditions favoring life on Earth are of interest for mankind. Stromatolites are considered exemplary for potential microbial life on other planets and provide unique archives to better understand environmental conditions favoring the evolution of life on Earth. A detailed understanding of favorable environmental conditions for early life on Early Earth opens new horizons for society and researchers to better understand environmental conditions of putative life in the search of life on other planets.

- What are the overall objectives?
The objectives of project ELEMIN are to determine the mineralization history of mineral phases in stromatolites, to compare the source(s) of elements in Proterozoic and Archean seawater and to reconstruct environments in which early life on Earth developed.

- Conclusions
1. Carbonates of the 1 billion year old ParanoÃ¡ Group stromatolites (Brazil) and the 3.34 billion years old Strelley Pool Formation stromatolites (Australia) are prime geochemical archives of ancient waters from which chemical sediments precipitated. Rare earth elements, radiogenic Sm-Nd and stable Cd isotopes show pristine geochemical compositions. In contrast, silica phases in the Strelley Pool stromatolites were formed during the post-depositional processes.
2. Stromatolites from the Strelley Pool Formation were formed in seawater that received variable elemental fluxes from high-temperature, hydrothermal fluids and from terrestrial weathering and erosion processes. Stromatolites from the ParanoÃ¡ Group were deposited under more oxidized atmospheric-hydrospheric conditions in an epicontinental lagoon and on the open shelf. Their elemental budget is exclusively derived from continental weathering.
3. A novel geochemical toolbox applied to stromatolites from the Mesoproterozoic ParanoÃ¡ Group stromatolites unravels different environmental conditions within individual layers of stromatolitic carbonates on centimeter-scale resolution, i.e. can be used to distinguish carbonate that formed at the seawater-mat interface from carbonate that was formed within the microbial mat. This unique geochemical combination may serve in future studies as prime tool to reconstruct biogeochemical conditions in ancient microbial mats of the earliest microbial communities on Earth.

Work performed

Project ELEMIN was conducted by MC-IF S. Viehmann, PhD with supervisor Prof. P. Meister and in collaboration with Prof. C. Koeberl at the University of Vienna. Dissemination and exploitation of data and research results strictly follow the Horizon 2020 guidelines. Research results of project ELEMIN were and will be published in internationally renowned high-impact journals (see below) and were presented at national, international, public conferences and during workshops and invited talks in China, Australia and Germany. All data are available at from the corresponding author upon request and are available within the manuscripts. A detailed overview describing the publications within project ELEMIN summarize the work performed during the course of this project:

Viehmann et al. (2018, Chem. Geol), Kraemer et al. (2019, Ore Geol. Rev.) & Viehmann (2018, Geoscience): These publications apply rare earth element (REE) and radiogenic isotope studies (Hf-Nd-Sr) to chemical sediments to exploit the potential of these geochemical tools to determine depositional conditions and source of elements in ancient fluids. These publications report the presence of high-temperature hydrothermal fluids in ancient depositional milieus and precisely determine the source of elements (mafic vs. felsic sources). Decoupled Hf-Nd isotopes in seawater further indicate that emerged continental crust has been available for weathering and erosion processes already 2.7 billion years ago.

Viehmann et al. (2019, Gond. Res.): Combined petrographic (SEM) and geochemical (trace element and stable Cd isotope compositions) investigations of dome-shaped and conophyton-type stromatolites from the Mesoproterozoic ParanoÃ¡ Group were deposited in an epicontinental lagoon restricted from the open ocean. The stromatolites received their elemental composition from weathering and erosion of local hinterland rocks and continental shelf in exchange with open seawater. For the first time, we reconstructed redox conditions in an ancient microbial mat at the centimeter scale. We reconstructed the (bio)geochemical metal cycling processes, and the availability of nutrients within the microbial mat ca. 1 billion year ago.

Viehmann et al. (submitted to Precamb. Res.): Carbonates of the 3.34 Ga old Strelley Pool stromatolites show the oldest Sm-Nd age measured so far in stromatolites. Silica phases in the stromatolites and adjacent crystal fan carbonates show Paleoproterozoic and Late Archean formation ages suggesting post-depositional alteration/ formation of these phases. Combined REE and Nd isotopes of stromatolitic carbonates indicate that continental weathering and hydrothermal activity both affected local seawater chemistry supporting microbial life on a shallow marine platform.

Meister et al. (in prep for Facies): Mapping campaigns, petrographic, mineralogical and stable C-O investigations of an alternating clastic-carbonate sedimentary succession in the Triassic German Basin provide new insight into the conditions and formation processes of authigenic carbonates in ancient evaporative settings. These sedimentary structures and isotopic compositions indicate primary precipitation of the dolomite in a large playa lake.

Final results

For the first time nutrient availability within a microbial mat was reconstructed at highest resolution (Viehmann et al., 2019, Gond. Res.). The findings open new horizons for the interdisciplinary fields of Geosciences and Biology to better understand marine microbial habitats and nutrient availability for microbial communities. The most recent study (Viehmann et al., submitted to Precamb. Res.) applies a similar approach to the oldest stromatolites on Earth to directly date mineral phases, reconstruct the depositional environment in which the stromatolites were formed, and determine the source and availability of elements in seawater 3.34 billion years ago. Thus, project ELEMIN unleashes unique insights into physico-chemical conditions prevailing in the earliest microbial habitats to better understand the environmental conditions in which life thrived and established, but also provides a better understanding for the public regarding the origin and evolution of microbial life on Earth.