Microbial Biosignatures Associated with Iddingsite in Hadal Basalts from the Southern Mariana Trench

Abstract Iddingsitization is an aqueous alteration that is known to take place in meteorites and continental basalts providing a potential habitat for microbial life. However, little is known about the exact mode by which this reaction occurs in the hadal seafloor and its implication for the deep subsurface biosphere. A comprehensive investigation of hadal basalts from the southern Mariana Trench (SMT) conducted with microscopic examinations shows that iddingsite occurs as augite‐hosted veins, rims, and mineral grains formed completely in place of augite within the SMT basalts. Carbon geochemistry indicates that organic matter with homogenous δ 13 C values between –27.8‰ and –27.2‰ might be biogenically accumulated in the SMT basalts. Furthermore, the close spatial relationships between carbonaceous matter (CM) and goethite in iddingsite point to microbial attachment to iddingsite minerals. Thus, iddingsitization might have fueled H 2 ‐utilizing microorganisms inhabiting the hadal oceanic crust, thereby leading to the formation of CM, as implied by oxygen isotopic compositions revealing low alteration temperatures (32–83°C) favorable for microbial growth. In all, microbial biosignatures associated with iddingsite in the SMT basalts are highlighted, and these results could pave the way for deciphering the deep subsurface biosphere at hadal zones.