Past endolithic life in metamorphic ocean crust

Abstract

The known deep subsurface biosphere on Earth persists in diversified habitats, including deep within igneous rocks of the oceanic crust. Here, we extend the range of the deep subsurface biosphere to metamorphic ocean crust of a subduction zone. We report fossilised life in zeolite facies rocks, which formed by low grade metamorphism, from the southern Mariana trench. Dense carbonaceous spheroids, filaments, and Frutexites-like structures are preserved in these rocks, which are enriched in organic carbon but depleted in 13 C. The distinct difference in the GDGT-0 vs. crenarchaeol and the branched vs. isoprenoid tetraether values between the inner and outer portions of these rocks indicate the in situ production of organic carbon. We demonstrate that these structures may result from the past activity of potential chemolithoautotrophs within the metamorphic crust, as implied by their morphologies, Raman spectra, carbon isotopes, and biomarker signatures, as well as the Fe oxidation state within whole rocks. We propose that fluid-rock reactions at temperatures within the tolerance of life during low grade metamorphism contributed to microbial subsistence within the biotope. The low grade metamorphic ocean crust of the subduction zone likely represents Earth's deepest, and one of its largest, microbial ecosystems, which may potentially influence the deep carbon cycle.

Geology, environment, and life in the deepest part of the world’s oceans

“…53 The episodic release of fluids from the subducting slab along the fault structure could lead to the formation of serpentinite mud volcanoes in the forearc region above the hadal trench. 4,54,55 Growing evidence 4,47,56,57 has shown that the hydration of the incoming plate, subsequent dehydration of the incoming slab, and hydration of the mantle wedge of the overriding plate have profound impacts on sedimentary, geochemical, and biological processes in the hadal trench (Figure 4): (1) the hydration and dehydration of the hadal lithosphere could support fluid discharge and fuel chemolithoautotrophic life on the hadal seafloor and within the hadal lithosphere, [58][59][60] (2) the dehydration of the incoming plate could trigger earthquakes that substantially influence the sedimentary flux and organic carbon supply to the hadal trench, 61,62 and (3) the hydration/ alteration-derived sediments could increase the supply of carbon, iron, and other elements to the hadal trench and influence biogeochemical cycling therein. 63 Hydration of mantle lithosphere likely supporting fluid discharge It is well known that hydration of mantle rocks can drive the serpentinitehosted hydrothermal system at the deep-sea floor, which vents warm (40-90 C), high-pH (9-11) fluids enriched in H 2 and CH 4 that fuel chemolithotrophic life.…”

“…A hypothesis that subduction zone zeolitefacies rock hosts Earth's deepest subsurface microbial ecosystems is proposed. 60 This is based on the fact that (1) zeolite-facies metamorphism starts at temperatures of approximately 40 C, well within the tolerance of life, and (2) zeolite-facies metamorphism can extend up to 14 km into the ocean crust of the subduction zone at temperatures below 121 C. To test this hypothesis, Peng et al 60 recovered zeolite-facies metamorphic rocks from the southern Mariana Trench at a water depth of 5,500 to 6,800 m using the manned submersible Jiaolong. Dense carbonaceous aggregates and filaments, which result from the past activity of chemolithoautotrophs, were observed to be preserved in those metamorphic rocks.…”

“…Growing evidence 4 , 47 , 56 , 57 has shown that the hydration of the incoming plate, subsequent dehydration of the incoming slab, and hydration of the mantle wedge of the overriding plate have profound impacts on sedimentary, geochemical, and biological processes in the hadal trench ( Figure 4 ): (1) the hydration and dehydration of the hadal lithosphere could support fluid discharge and fuel chemolithoautotrophic life on the hadal seafloor and within the hadal lithosphere, 58 , 59 , 60 (2) the dehydration of the incoming plate could trigger earthquakes that substantially influence the sedimentary flux and organic carbon supply to the hadal trench, 61 , 62 and (3) the hydration/alteration-derived sediments could increase the supply of carbon, iron, and other elements to the hadal trench and influence biogeochemical cycling therein. 63

Figure 4 Geological background of the hadal trench in the plate tectonic model This model integrates available information on the hydration of the hadal lithosphere 37 , 56 , the fluid activity, 55 , 59 and the carbon flux 64 in the subduction zone.

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“…A hypothesis that subduction zone zeolite-facies rock hosts Earth’s deepest subsurface microbial ecosystems is proposed. 60 This is based on the fact that (1) zeolite-facies metamorphism starts at temperatures of approximately 40°C, well within the tolerance of life, and (2) zeolite-facies metamorphism can extend up to 14 km into the ocean crust of the subduction zone at temperatures below 121°C. To test this hypothesis, Peng et al.…”

“…To test this hypothesis, Peng et al. 60 recovered zeolite-facies metamorphic rocks from the southern Mariana Trench at a water depth of 5,500 to 6,800 m using the manned submersible Jiaolong. Dense carbonaceous aggregates and filaments, which result from the past activity of chemolithoautotrophs, were observed to be preserved in those metamorphic rocks.…”