Search

The residence time of fluids circulating through deep‐sea hydrothermal systems influences the extent of water‐rock reactions and the flux of major and minor elements to the ocean. While the fluid residence times in numerous basaltic and gabbroic systems have been determined, those of the less studied ultramafic systems are currently unknown. Fluids that interact with mantle rocks have fundamentally different chemistries and therefore have unique influences on seawater chemistry. In this first investigation of radium isotopes in a serpentinite‐hosted system, vent fluids were discovered to contain 10–100 times greater activities of 223Ra (half‐life = 11.4 days) than observed in high‐temperature basalt‐hosted systems. The 223Ra activities of 10–109 dpm L−1 produce 223Ra/226Ra activity ratios ranging from 9 to 109. These extremely high 223Ra activities, which are accompanied by low activities of 226Ra, place significant constraints on fluid residence times and the adsorption coefficient of radium between fluid and rock. Our data constrain the nondimensional retardation factor (R) to very low values between 1 and 4, reflecting the extent to which Ra is transported more slowly than fluids due to adsorption and other processes. These results suggest that the residence time of fluids in contact with serpentinite is less than 2 y and perhaps as low as 0.5 y. They are surprisingly similar to those of basalt‐hosted systems. Thus, fluids in hydrothermal systems share similar hydrogeological characteristics despite differences in rock types, underlying porosity, and heat sources, enabling larger‐scale models of hydrothermal biogeochemistry to be developed across systems. Plain Language Summary: The well‐known "black smoker" hydrothermal systems occur at ocean spreading centers. High temperature (200°C–400°C) fluids in these systems pass through basalt, where they become both acidic and rich in metals. As they emerge on the seabed after a few years, they precipitate metal‐rich chimneys. Lesser‐known hydrothermal systems occur in serpentinite. These systems form as minerals found deep in the earth are uplifted to the seabed and exposed to seawater. Here chemical reactions at lower temperatures (100°C–200°C) occur that convert primary Fe‐bearing minerals to serpentine, releasing hydrogen (H2) and producing alkaline fluids. These conditions allow simple carbon molecules to be converted to more complex molecules including, potentially, amino acids, without the assistance of living organisms. Once formed, these molecules slowly degrade. A fundamental question regarding serpentinite‐hosted systems is whether the molecules can be exported to the seabed before they are destroyed. We discovered a rare radium isotope, 223Ra (half‐life = 11 days), was much enriched compared to the more common 226Ra (half‐life = 1,600 y) in fluids from the Lost City serpentinite‐hosted system. This argues fluid circulation times of less than 2 y, likely short enough to allow complex molecules to be exported into overlying ocean waters. Key Points: Lost City vent fluids are highly enriched in the short‐lived radium isotope, 223Ra, but not in longer‐lived 226Ra and 228RaRadium adsorption from Lost City vent fluids to solids is very low, similar to basalt‐hosted systemsResidence times of fluids in Lost City serpentinite are constrained to less than 2 y [ABSTRACT FROM AUTHOR]

We have developed a new method for the collection, purification, and measurement of natural levels of (32)P and (33)P in rain, marine particulates, and dissolved constituents of seawater. (32)P and (33)P activities were measured using a recently developed ultra-low-level liquid scintillation counter. Measurement by liquid scintillation counting allows, for the first time, simultaneous measurement of both (32)P and (33)P. Furthermore, (33)P activities are measured with high efficiency (>50%), regardless of the amount of stable phosphorus in the sample. Liquid scintillation also produces energy specific β spectra which has enabled us to identify previously unrecognized β-emitting contaminants in natural samples. In order to remove these contaminants, new methods of purification have been developed which utilize a series of precipitations and anion and cation exchange columns. Rainwater and dissolved seawater samples were extracted from large volumes of rain- and seawater, 5-20 and >5000 L, respectively, using iron-impregnated polypropylene filters. On these filters, it was possible to load between 25 and 30% Fe(OH)(3) by weight, over twice that loaded on previously utilized materials. Using our collection, purification, and liquid scintillation counting techniques, it was possible to obtain specific (32)P and (33)P activities with less than 10% error (2σ) in rainwater and 20% error (2σ) in seawater.