Reassessing the origin of organic compounds in Mariana forearc serpentinite mud volcanoes: insights from isotopologue and molecular signatures

Abstract Serpentinization-associated geofluid systems are thought to facilitate the abiotic synthesis of organic compounds, which may have played a key role in the prebiotic chemical evolution on the early Earth. The Mariana convergent margin hosts one of the serpentinization-associated systems, which has evolved into serpentinite mud volcanoes at the forearc region. Previous investigations have revealed abundant hydrocarbons and other organic compounds in the Mariana geofluid systems, but their origin, whether abiogenic or not, remains controversial owing to the potential input of organic-rich materials from the subducting slab. In this study, we collected serpentinite mud samples from two Mariana mud volcanoes, the South-Chamorro and Asùt Tesoru seamounts, using a deep-sea boring machine system, with the deepest core from 33.7 m below the seafloor. Consistent with prior scientific-drilling-based observations from the Ocean Drilling Program (ODP) Expedition 195 and the International Ocean Discovery Program (IODP) Expeditions 366, the high pH and negligible abundance of Mg coupled with patterns of major ions species in the deep parts of cores indicate that the upwelling pristine fluids were recovered successfully. To elucidate the origin of organics in the muds, the doubly-substituted (clumped) isotopologue of ethane ( 13 C 2 H 6 ) and conventional isotopic compositions of various hydrocarbons were analyzed. In addition, the characteristic patterns of long-chain alkanes, fatty acids, and amino acids were examined to assess the origin of organic compounds other than ethane. The clumped isotopologue signatures of ethane indicated the biotic source with thermal diagenesis. Similar signatures, indicating the thermal maturation of biotic organic matter, are further observed in the polyphasic analyzes: the high concentrations of ammonium and the predominance of even-numbered carbon chains in both aliphatic hydrocarbons with the unresolved complex mixture and long-chain fatty acids. These results indicate that a major part of the indigenous organic compounds in the Mariana serpentinite mud volcanoes is derived from thermogenic alteration of subducting organic matter rather than from abiotic synthesis. Although distinguishing the source of methane, the most abundant hydrocarbon, is challenging, its thermogenic provenance seems to be very likely because all other abundant organic matter represents the biotic origin that have undergone thermal maturation. Our study shows that multiple proxies are necessary critically to distinguish organic molecule sources; otherwise, the abiotic synthesis of organic matter in the Earth’s crust may be overestimated.