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Site Description

To understand serpentinization of ophiolites, which originate in the oceans, it is critical to understand the processes associated with seafloor serpentinization. The Lost City Hydrothermal Field, atop the Atlantis Massif near the Mid-Atlantic Ridge, is a well-studied example of a marine serpentinite-hosted ecosystem. At the LCHF, hydrothermal fluids rich in H₂ and CH₄ vent through tall calcium carbonate edifices that tower up to 60 m off of the seafloor. Over the past decade concerted studies of the geochemistry and microbiology of the LCHF have revealed that volatile rich fluids support expansive biofilms of methane-metabolizing Archaea (Schrenk et al., 2004; Brazelton et al., 2006) that may be supported by CH₄ cycling from deep (mantle) sources. Mineralogical and geochemical studies of seeps and subseafloor rocks have also suggested the importance of abiogenic formate as a mantle-derived carbon source supporting microbial communities (Lang et al., 2012) and have indicated sulfate to be an important oxidant and driver of microbial respiration in the subsurface (Lang et al., 2012; Alt and Shanks, 1998).

Geologic History

The Atlantis Massif is an oceanic core complex, a portion of lower crustal gabbroic rocks and peridotite mantle rocks uplifted during faulting of the seafloor (e.g. Ildefonse et al., 2007) that formed over the last 1-2 my (Blackman et al., 2002; Karson et al., 2006). Oceanic core complexes are most often associated with slow spreading ridges such as the Mid-Atlantic Ridge, which sits ~15 km east of the Atlantis Massif. The Lost City Hydrothermal Field sits on a terrace on the south wall of the massif (Figure 1 Kelley et al., 2007). The peridotite is highly serpentinized (70-100%) (Kelley et al., 2007) and it is the reaction of seawater and peridotite (e.g. serpentinization) that produces the high pH (up to 12), high H₂ and CH₄, moderate temperature (up to ~90C) fluids venting from the carbonate chimneys (Kelley et al., 2005) (Figure 2). The carbonate chimneys themselves are the result of the warm, reacting fluids coming into contact with cool, carbonate-bearing seawater and precipitating carbonate mineral phases (Kelley et al., 2007). While work has been done to investigate life in and on the carbonate chimneys (Schrenk et al., 2004; Brazelton et al., 2006), little is known about the life in the subsurface rocks of the massif itself (e.g. Mason et al., 2010).

IODP/ECORD Expedition 357

A seafloor drilling project, the , led by Gretchen Früh-Green (ETH-Zurich) and Beth Orcutt (Bigelow Marine Science Lab) and supported by both the European Consortium for Ocean Research Drilling (ECORD) and the International Ocean Â鶹ÊÓƵy Program (IODP) occurred in fall and winter 2015/2016. A seabed rock drill was used to obtain intact cores up to ~ 15m long from x boreholes. Borehole locations were chosen ‘to explore the extent and activity of the subsurface biosphere and assess how abiotic and biotic processes change with aging of the lithosphere and with variations in rock type… in an effort to better understand the role of serpentinization in driving hydrothermal systems, in sustaining microbiological communities, and in the sequestration of carbon in ultramafic rock’ (). Co-Is Schrenk, Brazelton, and Mayhew and RPL postdoc Katrina Twing are members of the expedition’s science party and will have immediate access to core samples for microbiological and geochemical analyses. All analyses will be coordinated and integrated with similar work at CROMO and Oman.