[B-CG06] Decoding the history of Earth: From Hadean to the present

[BCG06-06] MALDI-TOFMS 2D-mapping analysis of indigenous biomarkers for oxygenic photosynthesis preserved in the Archean and Proterozoic black shales

Kei-ichiro Ishikawa¹, Hiroki Saito¹, *Kosei E. Yamaguchi^1,2, Tomoyo Okumura³, Akira Ijiri⁴ (1.Toho University, 2.NASA Astrobiology Institute, 3.Kochi University, 4.Kobe University)

Keywords:Archean, Proterozoic, Biomarker, Black Shales

Biogenesity of carbonaceous, cyanobacteria-like materials found in the ~3.5Ga chert (e.g., Schopf, 1993) has been challenged, with an opposing interpretation of non-biological, post-depositional origin (e.g., Brasier et al., 2002). Authenticity of 2alpha-methylhopane, a likely biomarker of oxygenic photosynthesizers cyanobacteria, extracted from the 2.7Ga black shale (Brocks et al., 1999, Eigenbrode et al. 2008) has also been challenged (e.g., Brocks et al. 2011, Rasmussen et al. 2008). Therefore the timing of emergence for oxygenic photosynthesis remains unconstrained. It is important to determine if a “biomarker” in black shales is indigenous or contamination in later stages by a novel method.

Here we report the results of in situ biomarker mapping, by MALDI-TOFMS or imaging mass spectrometry (SHIMADZU iMScope), for Archean-Paleoproterozoic black shales from Australia and Ghana. The MALDI-TOFMS method has advantages over the traditional GC-MS method, where location information is lost during biomarker extraction from rocks that are powdered and homogenized for extraction by organic solvents.

We could observe 2D, sedimentary-structure-dependent distributions of hopane in thin sections made from drillcores of the black shales in the 3.2Ga and 2.7Ga black shales from Pilbara, Western Australia, and 2.2Ga black shales from Ghana. The 2.7Ga drillcores were used for detection of 2alpha-methylhopane by Brocks et al. (1999) and for geochemical and isotopic analyses by Yamaguchi (2002). We also used drillcores (ABDP#10) of the shallow-facies stromatolitic carbonates of the 2.7Ga Tumbiana Formation from the same district.

We confirmed syngenesity of hopane in all of the 3.2Ga, 2.7Ga, and 2.2Ga black shales, suggesting operation of oxygenic photosynthesis in the shallow ocean well before and after the inferred GOE. Biomarker mapping by MALDI-TOFMS proved to be a powerful tool for testing its syngenesity. Our results have important implications for the evolution of microbial biosphere and that of redox states in the atmosphere-ocean system in the early Earth.