An interdisciplinary team of scientists led by scientists from Utrecht University with participation from University of Gothenburg have found new clues about how deep life may extend into the Earth’s interior near the deepest place on our planet – the Challenger Deep in the Pacific Ocean. In their paper published in the Proceedings of the National Academy of Sciences, they examine complex organic molecules that have been trapped within rock fragments brought to the seafloor by massive mud volcanoes.
The massive mud volcanoes that sit above the Izu-Bonin-Mariana subduction zone, where the Pacific Plate is dragged under the Philippine Sea Plate, bring rock fragments from ~ 20 km depth up to the seafloor. Fuelled by cold fluids that are released as the down-going plate heats up, rocks deep below the mud volcano undergo chemical reactions with the fluids during a process called serpentinization. This process is affiliated with life at mid-ocean ridges and may feed microbial life that does not depend on light for its main energy source. In their hunt for tell-tale signs of life, the scientists have analysed rock fragments that have been carried by the mud volcanoes from as deep as 20 km within the Earth. Using state-of-the-art analytical techniques they found organic matter encapsulated within the clasts.
“You could think of this organic matter trapped within a rock a bit like a message in a bottle.” says Oliver Plümper, Earth scientist at Utrecht University and lead author of the paper.
Although Dr. Plümper and his colleagues cannot pinpoint the exact origin of the organic matter, chemical analysis of the constituents hint at microbial life deep within or below the mud volcano. This is consistent with calculations conducted by the authors using the currently known temperature limit for life, 122 °C, and the temperatures expected under the mud volcanoes, which suggest that life could exist as deep as 10,000
meters below seafloor. This is considerably deeper than other serpentinizing regions such as mid-ocean ridges and could have provided a sheltered habitat for life helping it to survive the more violent phases of Earth’s early history.
Oliver Plümper says “The mud volcanoes are a unique window into the deep subsurface and allow us to probe processes that are otherwise hidden from us. Finding the organic material within the rock clasts was very exciting as they may point to a deep biosphere below the mud volcanoes.”
PNAS – Subduction zone forearc serpentinites as incubators for deep microbial life
Significance
We document organic matter encapsulated in rock clasts from a oceanic serpentinite mud volcano above the Izu–Bonin–Mariana subduction zone (Pacific Ocean). Although we cannot pinpoint the exact origin of the organic matter, chemical analysis of the constituents resembles molecular signatures that could be produced by microbial life deep within or below the mud volcano. Considering the known temperature limit for life, 122 °C, and the subduction zone forearc geotherm where such mud volcanoes are located, we estimate that life could exist as deep as ∼10,000 m below the seafloor. This is considerably deeper than other active serpentinizing regions such as midocean ridges and could have provided sheltered ecosystems for life to survive the more violent phases of Earth’s history.
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–Bonin–Mariana 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, Mid-Atlantic 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.
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