Carbonate factories in the early Archean and their geobiological impacts

by Wanli Xiang
Doctoral thesis
Date of Examination:2023-06-30
Date of issue:2023-08-08
Advisor:Prof. Dr. Joachim Reitner
Referee:Prof. Dr. Joachim Reitner
Referee:Prof. Dr. Jan-Peter Duda
Persistent Address: http://dx.doi.org/10.53846/goediss-10047

 

 

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Abstract

English

Paleoarchean carbonates in the Pilbara Craton (Western Australia) are important archives for early life and environment on early Earth. Amongst others, carbonates occur in interstitial spaces of ca. 3.5–3.4 Ga pillow basalts (North Star-, Mount Ada-, Apex-, and Euro Basalt, Dresser Formation) and associated with bedded deposits (Dresser- and Strelley Pool Formation, Euro Basalt). This study aims to understand the formation and geobiological significance of those early Archean carbonates by investigating their tempo-spatial distribution, petrography, mineralogy, and geochemistry (e.g., trace elemental compositions, δ13C, δ18O, 87Sr/86Sr). Three carbonate factories are recognized: (i) an oceanic crust factory, (ii) an organo-carbonate factory, and (iii) a microbial factory. In case of all three carbonates factories, hydrothermal fluids play a key-role in the formation and preservation of precipitates. For instance, alkaline earth metals and organic materials delivered by fluids may promote carbonate precipitation, whilst soluble silica in the fluids drives early chert formation, delicately preserving authigenic carbonate precipitates and associated features. The implications are manifold: Carbonates of the microbial factory provide valuable insights into the interplay between geological processes and early microbial life, while geochemical information encoded in pristine interstitial carbonates of the oceanic crust factory provide a baseline reference for the early Archean ocean water composition. For example, the geochemical characteristics (REE+Y, δ13C, 87Sr/86Sr) of the pristine interstitial carbonates from Dresser Formation and Apex Basalt are indicative of seawater and seawater-derived low-temperature hydrothermal fluid at 3.5 Ga. They can be excellent materials for reconstructing ocean temperature and studying seawater-oceanic crust interaction. Regardless of the formation pathway, the Paleoarchean carbonates might have been major carbon sinks on the early Earth, thus modulating climate variability and, by extension, habitability.
Keywords: Early life; habitability; bio-nutrients; hydrothermal alteration; biomineralization; organo-mineralization
 
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