Fluid mixing and the deep biosphere of a fossil Lost City-type hydrothermal system at the Iberia Margin

Abstract

Subseafloor mixing of reduced hydrothermal fluids with seawater is believed to provide the energy and substrates needed to support deep chemolithoautotrophic life in the hydrated oceanic mantle (i.e., serpentinite). However, geosphere-biosphere interactions in serpentinite-hosted subseafloor mixing zones remain poorly constrained. Here we examine fossil microbial communities and fluid mixing processes in the subseafloor of a Cretaceous Lost City-type hydrothermal system at the magma-poor passive Iberia Margin (Ocean Drilling Program Leg 149, Hole 897D). Brucite−calcite mineral assemblages precipitated from mixed fluids ca. 65 m below the Cretaceous paleo-seafloor at temperatures of 31.7 ± 4.3°C within steep chemical gradients between weathered, carbonate-rich serpentinite breccia and serpentinite. Mixing of oxidized seawater and strongly reducing hydrothermal fluid at moderate temperatures created conditions capable of supporting microbial activity. Dense microbial colonies are fossilized in brucite−calcite veins that are strongly enriched in organic carbon (up to 0.5 wt.% of the total carbon) but depleted in 13 C (δ 13 C TOC = −19.4‰). We detected a combination of bacterial diether lipid biomarkers, archaeol, and archaeal tetraethers analogous to those found in carbonate chimneys at the active Lost City hydrothermal field. The exposure of mantle rocks to seawater during the breakup of Pangaea fueled chemolithoautotrophic microbial communities at the Iberia Margin, possibly before the onset of seafloor spreading. Lost City-type serpentinization systems have been discovered at midocean ridges, in forearc settings of subduction zones, and at continental margins. It appears that, wherever they occur, they can support microbial life, even in deep subseafloor environments.serpentinization | microfossils | lipid biomarkers | brucite−calcite | passive margin

Subduction zone forearc serpentinites as incubators for deep microbial life

“…Abiotic routes have been demonstrated to produce hydrocarbons and amino acids under hydrothermal conditions akin to serpentinization (29), but it is unclear whether these processes can produce the organic complexity observed in the serpentinite clasts. Intriguingly, similar Raman spectra to those reported here have been documented in other serpentinites and correlate with spectra taken from bacteria (6) and the presence of bacterial lipid biomarkers (7). Moreover, the acquired Raman spectra (Fig.…”

“…Thus, nanosized alloys could have facilitated CH 4 production over geologically relevant timescales below the upper temperature limit for life (122°C). In near-surface serpentinizing systems, hydrothermal fluids can mix with, for example, seawater, resulting in disequilibria that may provide the energy and substrates needed to support chemolithoautotrophic life (7). In contrast, a slowly ascending serpentinite mud along deep-reaching forearc faults may allow the system to remain much closer to equilibrium, regulating the activities of H 2 , CH 4 , CO 2 , and the Fe(II)/Fe(III) ratio in the solids and fluids limiting energy sources.…”

“…Although serpentinization leads to extreme pH conditions and limited nutrient and electron acceptor availability (1), microgenomic studies of serpentinization-fueled hydrothermal deep-sea vents and continental fluid seeps show evidence for microbial H 2 and CH 4 utilization (1,(3)(4)(5). Furthermore, micrometer-sized organic matter has been detected in dredged seafloor serpentinites (6) and in subseafloor mixing zones between seawater and serpentinization-derived fluid (7). The former study suggests that serpentinization-fueled microbial communities may use solid electron acceptors, particularly ferric iron from magnetite (Fe 2 (Fig.…”

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex, Spain)

“…In slow spreading mid‐ocean ridges (Figure a), serpentinite‐hosted carbonate–silicate rocks occur in the stockwork of submarine low‐ T hydrothermal fields (Lafay et al., ; Ludwig, Kelley, Butterfield, Nelson, & Früh‐Green, ; Schwarzenbach, Früh‐Green, Bernasconi, Alt, & Plas, ), in detachment faults of oceanic core complexes (Bach et al., ; Picazo, Manatschal, Cannat, & Andréani, ; Schroeder et al., ) and in transform faults (Alt et al., ; Bonatti, Lawrence, Hamlyn, & Breger, ). In these settings, carbonates are heterogeneously distributed at different scales in veins, replacing serpentine mesh textures and as matrix cement of tectonic or sedimentary serpentinite breccias (Figure a; Bonatti et al., ; Grozeva, Klein, Seewald, & Sylva, ; Klein et al., ; Lafay et al., ; Picazo et al., ; Schroeder et al., ). Infiltration of seawater into deeply rooted faults in intermediate‐ to fast‐spreading oceanic crust and faults formed during bending of the slab in the outer rise of subduction zones (Ranero, Phipps Morgan, McIntosh, & Reichert, ) might also result in serpentinization and carbonation of sub‐crustal peridotite (Figure b).…”

Geochemical bioenergetics during low‐temperature serpentinization: An example from the Samail ophiolite, Sultanate of Oman

“…[] suggest that sulfate‐reducing bacteria may be the source for some of the diether lipids they detected in Lost City samples that appear to not have archaeal origins, and Klein et al . [] provide fossil evidence for similar lipids in altered rocks from the Ibera margin that once hosted serpentinizing ecosystems.…”