[S-MP27] Evolution of the early continents and the oceans understood by multi-geological proxies

[SMP27-P04] Microbial TiO ₂ segregation for UV protection: Evidence from 2.72 Ga stromatolite

* サシダラン キラン¹、Madhusoodhan Satish-kumar¹、Nakamura Yoshihiro²、Ohfuji Hiroaki³ (1.Niigata University、2.AIST, Tsukuba、3.Tohoku University)

キーワード：Archean、Titanite Microtubes、UV protection、Stromatolite、trace fossil

The titanite filled microtubular structures often regarded as Earth's oldest trace fossil, remains as subject of debate. In this study, we present the first report of similar structures in a 2.72 Ga stromatolite from the Tumbiana Formation, Pilbara Craton. The stromatolite with ¹³ C depleted carbonaceous material (δ ¹³ C: –47.3‰) indicative of methanogenic microbial process, has undegone only low-grade metamorphism, as determined by Raman spectra of carbonaceous material thermometry (301 ± 6 °C). The titanite microtubes exhibit curved shape with pyrite at the tip, side branching with varying diameter relative to the main tube. Adittionally, their starbust-like pattern highly resembles the structures formed by ambient inclusion trail (AIT) mechanism. Electron backscattered diffraction and transmission electron microscopy analyses reveal a uniform single crystal orientation of titanite, irrespective of growth direction.Notably, the absence of significant titanium bearing minerals near these structures raises intriguing questions regarding the source of Ti. The presence of TiO₂ nanocrystals (likely anatase) adjacent to titanite microtubes suggests a possible microbial role TiO₂ precipitation. Supporting evidence includes the strong association of Ti content with CM and the presence of nano-scale hydroxyl channels hosting CM. These finding suggests that microorganisms may have induced TiO₂ precipitation as a defense mechanism to shield against harmful UV radiation in the harsh conditions of the Archean Earth.