[B-BG01] 地球惑星科学　生命圏フロンティア

[BBG01-01] Early magnetotaxis and the emergence of biomineralization

*Joseph Kirschvink^1,2,8、Noah C. Tashbook¹、Rory Changleng³、Lot Koopmans⁴、Myriam A.C. Kars⁵、Hironori Hidaka⁶、Kotaro Kawai⁷、Atsuko Kobayashi^2,8、Yuhji Yamamoto² (1.Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA、2.Kochi Core Center, Kochi University, B200 Monobe, Nankoku, Kochi 783-8502, Japan、3.Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA、4.Department of Earth Sciences, University of Oxford, Oxford, UK、5.International Ocean Discovery Program, Texas A&M University, College Station, TX 77845, USA、6.Department of Systems Control Engineering, Tokyo Institute of Technology, Meguro, Japan、7.Department of Mechanical Engineering, Tokyo Institute of Technology, Meguro, Japan、8.Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Japan)

キーワード：Magnetotactic Bacteria, Scanning Magnetic Imaging, Magnetofossils, First-Order Reversal Curves (FORCs)

Magnetotaxis is an evolutionarily advantageous form of microbial locomotion, thought to have evolved well before the Great Oxidation Event according to the genetic profile of magnetosome gene clusters. We report here rock magnetic data from remarkably well preserved, shallow-water black chert samples from the Middle Marker Bed of the Hooggenoeg Formation from the Barberton greenstone belt of South Africa, dated to 3.47 Ga. We used a series of novel magnetometric techniques to test for the possible presence of bacterial magnetofossils embedded in an unusually and exceptionally transparent quartz (SiO₂) matrix. Of particular interest is an otherwise ordinary-looking black chert layer, which upon examination in thin section is actually a microbreccia of reduced carbonaceous fragments suspended within this clear matrix; we interpret these as disrupted and redeposited fragments of an early-silicified algal mat in an ocean supersaturated with respect to sodium silicate. SEM examination of the chert reveals a grain size < 0.5 µm, which, along with Raman spectroscopy argue for considerably low peak metamorphic temperatures. First-order reversal curves (FORCs) indicate single-domain, non-interacting magnetic nanoparticles, consistent with magnetofossil chains. Scanning SQUID imaging of this unit demonstrates that most of the remanent magnetization is located in a small subset of the carbonaceous specks, suggesting a magnetic stratification in the original algal mat. This is a known feature of magnetotactic bacteria in extant biofilms, and makes those bits compelling targets for future TEM/FIB examination. Taken collectively, these results point to the presence and longevity of magnetotactic bacteria in Archean marine environments, with significant implications for the initial evolutionary development of microbial biomineralization processes and understanding the early fluctuations of Earth’s magnetic field.