Subduction zone forearc serpentinites as incubators for deep microbial life

Abstract

Serpentinization-fueled systems in the cool, hydrated forearc mantle of subduction zones may provide an environment that supports deep chemolithoautotrophic life. Here, we examine serpentinite clasts expelled from mud volcanoes above the Izu-BoninMariana subduction zone forearc (Pacific Ocean) that contain complex organic matter and nanosized Ni-Fe alloys. Using time-of-flight secondary ion mass spectrometry and Raman spectroscopy, we determined that the organic matter consists of a mixture of aliphatic and aromatic compounds and functional groups such as amides. Although an abiotic or subduction slab-derived fluid origin cannot be excluded, the similarities between the molecular signatures identified in the clasts and those of bacteria-derived biopolymers from other serpentinizing systems hint at the possibility of deep microbial life within the forearc. To test this hypothesis, we coupled the currently known temperature limit for life, 122°C, with a heat conduction model that predicts a potential depth limit for life within the forearc at ∼10,000 m below the seafloor. This is deeper than the 122°C isotherm in known oceanic serpentinizing regions and an order of magnitude deeper than the downhole temperature at the serpentinized Atlantis Massif oceanic core complex, MidAtlantic Ridge. We suggest that the organic-rich serpentinites may be indicators for microbial life deep within or below the mud volcano. Thus, the hydrated forearc mantle may represent one of Earth's largest hidden microbial ecosystems. These types of protected ecosystems may have allowed the deep biosphere to thrive, despite violent phases during Earth's history such as the late heavy bombardment and global mass extinctions.deep biosphere | serpentinization | subduction zone | forearc

Submicron Raman spectroscopy mapping of serpentinite fault rocks

“…However, the peaks in lizardite and chrysotile have an identical shape, overlap with other silicate minerals, and their positions tend to shift as shown by the variations reported in the literature (Figure S2). Antigorite also has a weak, unique peak at 1,045 cm −1 , but this has little utility in high‐throughput mapping …”

“…The spectra and dominant peaks shown in Figure are consistent with published literature wavenumber values (Figure S2). The hydroxyl peak positions may vary by ±2 wavenumbers between samples—this is likely due to local cation substitutions . These low levels of substitution do not manifest as additional bands but cause minor distortions in the crystal lattice and slightly alter the energetics of vibrational modes .…”

Distributions and Sources of Glycerol Dialkyl Glycerol Tetraethers in Sediment Cores From the Mariana Subduction Zone

“…Although it is not possible to unambiguously identify the origin of organic matter, stable carbon isotope of total organic matter has been used as an indicator of carbon sources and biogeochemical cycling in trench settings (Luo et al, ). Several studies have taken into account organic matter potential sources from the extreme hadal environments including (1) terrestrial transport, (2) photoautotrophic primary production in the ocean surface, and (3) in situ chemoautotrophic carbon fixation (Burd et al, ; Gallo et al, ; Plümper et al, ; Tarn et al, ). It is important to note that δ 13 C and C/N values together are able to differentiate between terrestrial and marine phytoplanktonic sources of organic matter (Luo et al, ).…”

Geological and Geochemical Constraints on the Origin and Evolution of Life

“…Contact of the fluid with air causes calcium carbonate species to plate out. Serpentinite existed by *3.85 Ga in the oldest rocks where evidence of it and its exposed mantle protolith might reasonably exist (Friend et al, 2002;Nutman et al, 2009;Pons et al, 2011;Pope et al, 2012;Plümper et al, 2017).…”

“…Life exists at higher pressures but at low temperatures in the deep ocean. Forearcs with their low heat flow may be habitable down to 10 km below the seafloor (Plümper et al, 2017).…”