[B-CG04] Decoding the history of Earth: From Hadean to the present

[BCG04-09] Nanoscale analysis of trace elements in the Gunflint microfossils using a NanoSIMS: implication for the elemental cycle in the Paleoproterozoic biosphere

*Akizumi Ishida¹, Kouhei Sasaki¹, Takeshi Kakegawa¹, Naoto Takahata², Yuji Sano² (1.Department of Earth Science, Graduate school of Science, Tohoku University, 2.Atmosphere and Ocean Research Institute, the University of Tokyo)

Keywords:Gunflint Microfossils, Paleoproterozoic, NanoSIMS, phosphorus

In this study, we focused on elemental distributions of the Paleoproterozoic ‘Gunflint microfossil’ by using lateral high-resolution secondary ion mass spectrometry (NanoSIMS). Distributions of bioessential trace elements, such as phosphorus, iron, and molybdenum were newly measured together with biological major elements (such as carbon, nitrogen, and sulfur). The ‘Gunflint microfossil’ is a generic term for the group of microbial fossils found in the Gunflint Formation, Canada, which is a Paleoproterozoic sedimentary formation. The Gunflint microfossil has several types of morphology with size distributions from a few microns to a hundred-micron meter. Their morphological characteristics with carbon isotope compositions or distributions of organic elements determined by SIMS were reported in previous studies (e.g., Tyler & Barghoorn, 1954, Science; House et al., 2000, Geology; Wacey et al., 2013, PNAS; Williford et al., 2013, GCA). Although microscale analyses of major organic elements were performed in previous studies, bioessential trace elements that have crucial information to determine not only metabolic pathways and species of microfossils but also further implicate the elemental cycle in the biosphere at that time, have rarely been measured because of some technical difficulties.
In our research, we adopted and improved the acid treatment technic proposed by Grey and Sugitani (2009, Precam.Res.), and finally succeed to isolate ‘naked’ Gunflint microfossils from bulk cherty rocks without any destructions of microfossil structures (Sasaki et al., 2021, in prep.). In this research, we developed a new methodology to measure the depth-dependent elemental profile utilizing the sputtering effect of primary ion beam by NanoSIMS targeting such ‘naked’ microfossils without any matrix effect from minerals. As a result, we succeed to detect phosphorus and trace amounts of molybdenum directly from the organic part of Gunflint microfossils. This is the first report of the detection of organic phosphorus from the Paleoproterozoic microfossils and suggests that microbes in 1.9 billion years ago have already utilized phosphorus similar to modern life. This further implicates that life in this age had already been evolved and utilized phosphorus-based bio-structures and functions such as phospholipid membrane or ATP. In addition, detection of organic-related molybdenum from microfossils, which is the typical central metal of the nitrogen metabolizing enzyme, has the potential to prescribe a microbial activity and surrounding biosphere in that age.