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The lithified lower oceanic crust is one of Earth’s last biological frontiers as it is difficult to access. It is challenging for microbiota that live in marine subsurface sediments or igneous basement to obtain sufficient carbon resources and energy to support growth1–3 or to meet basal power requirements4 during periods of resource scarcity. Here we show how limited and unpredictable sources of carbon and energy dictate survival strategies used by low-biomass microbial communities that live 10– 750 m below the seafloor at Atlantis Bank, Indian Ocean, where Earth’s lower crust is exposed at the seafloor. Assays of enzyme activities, lipid biomarkers, marker genes and microscopy indicate heterogeneously distributed and viable biomass with ultralow cell densities (fewer than 2,000 cells per cm3). Expression of genes involved in unexpected heterotrophic processes includes those with a role in the degradation of polyaromatic hydrocarbons, use of polyhydroxyalkanoates as carbon-storage molecules and recycling of amino acids to produce compounds that can participate in redox reactions and energy production. Our study provides insights into how microorganisms in the plutonic crust are able to survive within fractures or porous substrates by coupling sources of energy to organic and inorganic carbon resources that are probably delivered through the circulation of subseafloor fluids or seawater. 

International Ocean Discovery Program (IODP) Expedition 362T was part of the transit from the April–June 2016 tie up in Cape Town, South Africa, to the IODP Expedition 362 port call in Colombo, Sri Lanka, (4 July–6 August 2016). Hole U1473A remedi- ation operations, approved in March 2016 by the JOIDES Resolution Facility Board (JRFB), took place 12–21 July 2016. The objectives of the Expedition 362T remedial operations were to remove the me- chanical bit release retainer sleeve (MBR-RS) left at the bottom of Hole U1473A at the end of IODP Expedition 360, cement multiple fault zone intervals to stabilize them, obtain a borehole temperature log across the fault zones (at the beginning of operations), and deepen the hole by coring an interval of no more than ~20 m. The planned temperature logging run at the beginning of opera- tions was only partially successful because the logging tool could not be lowered below a ledge at 277 m wireline log depth below sea- floor (WSF). Subsequent reaming using two tricone bit runs estab- lished a clean hole free of debris to the total depth of 789.7 m drilling depth below seafloor (DSF) established during Expedition 360. The fishing run with the reverse circulation junk basket (RCJB) yielded a surprise: it deepened Hole U1473A by 0.5 m and no junk was present at the bottom of the hole (i.e., the MBR-RS must have been removed with the last RCJB run during Expedition 360 and fallen to the seafloor without leaving any operational evidence). Next, four coring intervals deepened Hole U1473A by another 19.2 m, recovering 16.55 m (86%). The last task, cementing four fault zones identified in cores and mapped precisely based on Expedition 360 wireline logs, was partly successful. We completely cemented the lowermost fault zone (584–500 m DSF) and partly cemented the second lowest and most intense fault zone (489–443 m DSF). The upper two, less severe fault zones were not cemented at all. 

This is the site description for U1473 at Atlantis Bank 

International Ocean Discovery Program (IODP) Expedition 360 was the first leg of Phase I of the SloMo (shorthand for “The nature of the lower crust and Moho at slower spreading ridges”) Project, a multiphase drilling program that proposes to drill through the out- ermost of the global seismic velocity discontinuities, the Mohor- ovičić seismic discontinuity (Moho). The Moho corresponds to a compressional wave velocity increase, typically at ~7 km beneath the oceans, and has generally been regarded as the boundary be- tween crust and mantle. An alternative model, that the Moho is a hydration front in the mantle, has recently gained credence upon the discovery of abundant partially serpentinized peridotite on the seafloor and on the walls of fracture zones, such as at Atlantis Bank, an 11–13 My old elevated oceanic core complex massif adjacent to the Atlantis II Transform on the Southwest Indian Ridge. Hole U1473A was drilled on the summit of Atlantis Bank during Expedition 360, 1–2 km away from two previous Ocean Drilling Program (ODP) holes: Hole 735B (drilled during ODP Leg 118 in 1987 and ODP Leg 176 in 1997) and Hole 1105A (drilled during ODP Leg 179 in 1998). A mantle peridotite/gabbro contact has been traced by dredging and diving along the transform wall for 40 km. The contact is located at ~4200 m depth on the transform wall be- low the drill sites but shoals considerably 20 km to the south, where it was observed in outcrop at 2563 m depth. Moho reflections, how- ever, have been found at ~5–6 km beneath Atlantis Bank and <4 km beneath the transform wall, leading to the suggestion that the seis- mic discontinuity may not represent the crust/mantle boundary but rather an alteration (serpentinization) front. This in turn raises the interesting possibility that methanogenesis associated with ser- pentinization could support a whole new planetary biosphere deep in the oceanic basement. The SloMo Project seeks to test these hy- potheses at Atlantis Bank and evaluate the processes of natural car- bon sequestration in the lower crust and uppermost mantle. A primary objective of SloMo Leg 1 was to explore the lateral variability of the stratigraphy established in Hole 735B. Comparison of Hole U1473A with Holes 735B and 1105A allows us to demon- strate a continuity of process and complex interplay of magmatic ac- cretion and steady-state detachment faulting over a time period of ~128 ky. Preliminary assessment indicates that these sections of lower crust are constructed by repeated cycles of intrusion, repre- sented in Hole U1473A by approximately three upwardly differenti- ated hundreds of meter–scale bodies of olivine gabbro broadly similar to those encountered in the deeper parts of Hole 735B. Specific aims of Expedition 360 focused on gaining an under- standing of how magmatism and tectonism interact in accommo- dating seafloor spreading, how magnetic reversal boundaries are expressed in the lower crust, assessing the role of the lower crust and shallow mantle in the global carbon cycle, and constraining the extent and nature of life at deep levels within the ocean lithosphere. 

Expedition reports: core descriptions, maps, data reports 

During the 11 day transit from Colombo, Sri Lanka, to International Ocean Discovery Program (IODP) Site U1473 at Atlantis Bank, the Expedition 360 science party reexamined cores drilled during Ocean Drilling Program (ODP) Leg 179 in Hole 1105A (Pettigrew, Casey, Miller, et al., 1999; Casey et al., 2007). This activity involved rigorously describing the cores and many of the accompanying thin sections with the primary purpose of familiarizing the science party with the material likely to be encountered at the new Site U1473, situated 1.4 km to the north. The science party developed templates for description of the igneous, metamorphic, and structural features of the cores and analyzed thin sections made during Leg 179 to establish core description protocols for the new Site U1473 cores. An additional benefit of redescribing Hole 1105A cores is that the data generated are in a format directly comparable with those for Hole U1473A. In general, our findings were very similar to those produced by the Leg 179 scientists; however, with a larger science party to work on the cores, some of the information collected is new. We include this information in this chapter, as a basis for direct comparison with the results of drilling at Site U1473. In addition, we were able to make certain physical properties measurements on Hole 1105A cores, including magnetic susceptibility measurements of core section halves, that had not been possible during Leg 179. It is important to note here that the observations made on Hole 1105A cores by the Expedition 360 science party augment rather than replace those made by the Leg 179 scientists. 

International Ocean Discovery Program (IODP) Expedition 360 was the first leg of Phase I of the SloMo (shorthand for “The nature of the lower crust and Moho at slower spreading ridges”) Project, a multiphase drilling program that proposes to drill through the outermost of the global seismic velocity discontinuities, the Mohorovičić seismic discontinuity (Moho). The Moho corresponds to a compressional wave velocity increase, typically at ~7 km beneath the oceans, and has generally been regarded as the boundary between crust and mantle. An alternative model, that the Moho is a hydration front in the mantle, has recently gained credence upon the discovery of abundant partially serpentinized peridotite on the seafloor and on the walls of fracture zones, such as at Atlantis Bank, an 11–13 My old elevated oceanic core complex massif adjacent to the Atlantis II Transform on the Southwest Indian Ridge. Hole U1473A was drilled on the summit of Atlantis Bank during IODP Expedition 360, 1–2 km away from two previous Ocean Drilling Program (ODP) holes: Hole 735B (drilled during ODP Leg 118 in 1987 and ODP Leg 176 in 1997) and Hole 1105A (drilled during ODP Leg 179 in 1998). A mantle peridotite/gabbro contact has been traced by dredging and diving along the transform wall for 40 km. The contact is located at ~4200 m depth at the drill sites but shoals considerably 20 km to the south, where it was observed in outcrop at 2563 m depth. Moho reflections have, however, been found at ~5–6 km beneath Atlantis Bank and <4 km beneath the transform wall, leading to the suggestion that the seismic discontinuity may not represent the crust/mantle boundary but rather an alteration (serpentinization) front. This then raises the interesting possibility that a whole new planetary biosphere may thrive due to methanogenesis associated with serpentinization. The SloMo Project seeks to test these two hypotheses at Atlantis Bank and evaluate carbon sequestration in the lower crust and uppermost mantle. A primary objective of SloMo Leg 1 was to explore the lateral variability of the stratigraphy established in Hole 735B. Comparison of Hole U1473A with Holes 735B and 1105A allows us to demonstrate a continuity of process and complex interplay of magmatic accretion and steady-state detachment faulting over a time period of ~128 ky. Preliminary assessment indicates that these sections of lower crust are constructed by repeated cycles of intrusion, represented in Hole U1473A by approximately three upwardly differentiated hundreds of meter–scale bodies of olivine gabbro broadly similar to those encountered in the deeper parts of Hole 735B. Specific aims of Expedition 360 focused on gaining an understanding of how magmatism and tectonism interact in accommodating seafloor spreading, how magnetic reversal boundaries are expressed in the lower crust, assessing the role of the lower crust and shallow mantle in the global carbon cycle, and constraining the extent and nature of life at deep levels within the ocean lithosphere.