Activities of ²²³Ra and ²²⁶Ra in Fluids From the Lost City Hydrothermal Field Require Short Fluid Residence Times

Abstract

Seawater passing through hot oceanic crust initiates a series of reactions that play a significant role in controlling the concentrations and distributions of major ions and trace metals in the ocean. The chemistry of these hydrothermal fluids depends on multiple variables including rock type, temperature, degree of prior rock alteration, convective path length, the effective water/rock ratio, and flow rate (Kadko, 1996;Von Damm et al., 1985). The vast majority of identified marine hydrothermal systems are hosted in mafic rocks (basalt and gabbro), resulting in acidic fluids that are rich in metals. In contrast, fluids that pass through ultramafic rocks (peridotite, including harzburgite and dunite) trigger a water-rock reaction called serpentinization that converts peridotite to serpentinite (a rock composed of one or more serpentine group minerals) and releases free hydrogen (H 2 ) to high pH fluids. High concentrations of H 2 coupled with appropriate catalysts, can lead to the nonbiological synthesis of organic molecules including methane, formate, and short-chain hydrocarbons (

Metabolic strategies shared by basement residents of the Lost City hydrothermal field

“…Alkaline hydrothermal fluids flow from the serpentinite basement and throughout the Poseidon structure, exiting at multiple locations across the field. The differing flow paths that lead to each location have distinct residence times (Moore et al, 2021) and produce distinct chemical and microbiological compositions of the venting fluids (Lang et al, 2012, 2018, 2021; Aquino et al, In Revision).…”

Timing Earth's Abiotic Kitchen: Short Hydrothermal Fluid Residence Times in Serpentinizing Oceanic Crust

“…We do not presently know, however, on what timeframe organic nitrogen molecules can be created -if at all -under realistic physico-chemical (and now temporal) constraints imposed by Lost City fluid compositions. Experimental amino acid synthesis (Moore et al, 2021). Reaction timescales are calculated at temperatures resembling the likely range of subsurface fluids at Lost City (∼150-300°C), and fluid pressures of 35 MPa (i.e., single-phase solutions).…”

“…Writing in Journal of Geophysical Research: Oceans, Moore et al (2021) recently expanded the application of these short-lived Ra radioisotope "timers" to constrain residence times of fluids venting at Lost City. Contrasting strongly with mafic-hosted hydrothermal systems, Moore et al show that the Lost City fluids display extraordinary enrichments in short-lived 223 Ra (t 1/2 = 11.4 days)-indeed, the highest measured to date in any oceanic setting.…”

“…Much research therefore remains to be done to constrain abiotic organic carbon and nitrogen production in these systems. Moore et al (2021) have, nonetheless, placed a timely and crucial "kitchen timer" constraint on the Lost City serpentinite aquifer, albeit predicated on a number of assumptions regarding the subsurface that need further confirmation. It will be interesting to observe if similarly short timescales are upheld by short-lived 223 Ra data from other ultramafic-hosted vent fluids (e.g., Mid-Cayman Rise, Frankle et al, 2020).…”

“…Figure 1. Timescales in which selected organic reactions have been demonstrated to occur in experimental hydrothermal fluids, compared to the 0.5-2 years residence time calculated for Lost City fluids(Moore et al, 2021). Reaction timescales are calculated at temperatures resembling the likely range of subsurface fluids at Lost City (∼150-300°C), and fluid pressures of 35 MPa (i.e., single-phase solutions).…”