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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]

Anthropogenically mediated ocean acidification (OA) has negative impacts on many marine organisms, especially calcifiers. However, systematic measurements of OA have only been made over the past four decades. In order to improve future predictions and understand how ongoing OA compares to natural variability on longer timescales, it is critical to extend records beyond observational time series. In the Cariaco Basin, located in the tropical Atlantic, near‐surface dissolved inorganic carbon reflects atmospheric carbon dioxide concentrations (CO2) since the Industrial Revolution, making it an ideal site for examining longer‐term variability. We extend the record of Cariaco Basin near‐surface [CO32−] back to 1240 CE, using the area density (shell weight (μg)/shell area (μm2)) of the planktonic foraminifer Globigerinoides ruber (pink). Multidecadal variability is observed throughout the record. Since the Industrial Revolution (1760–2007 CE), [CO32−] has declined by 0.22 μmol kg−1 year−1, in agreement with the magnitude and direction of change captured in the shorter instrumental time series. During the Little Ice Age (1500–1760 CE), a period marked by regional drought, substantial variability but no long‐term trend is observed, while a decrease in [CO32−] of 0.11 μmol kg−1 y−1 occurs at the end of the Medieval Climate Anomaly (MCA) (1240 – 1500 CE). Both the MCA and Little Ice Age contain substantial natural variability in near surface [CO32−] that we attribute to changes in regional upwelling and atmospheric CO2. However, the decline in [CO32−] occurring in the Post‐Industrial Period is anomalous against a backdrop of 800 years of natural variability, reflecting OA associated with anthropogenic increases in atmospheric CO2. Key Points: Foraminiferal area density serves as a proxy for paleo‐[CO32−] in the Cariaco Basin showing a 54 μmol kg−1 decline since the Industrial RevolutionLong‐term rate of [CO32−] change in the Post‐Industrial (1760–2007) Cariaco Basin has been double that which occurred during the Medieval Climate AnomalyCariaco Basin [CO32−] variability prior to the Industrial Revolution (1760 CE) is characterized by short‐term oscillations [ABSTRACT FROM AUTHOR]

Total particulate phosphorus (TPP), particulate inorganic P (PIP), and particulate organic P (POP) concentrations were measured in a year-long series of sediment trap samples collected throughout the oxic-anoxic water column (275 m, 455 m, 930 m, and 1,255 m) of the Cariaco Basin. TPP, PIP, and POP fluxes varied seasonally and decreased significantly with depth, from 65 to 19 µmol TPP m-2 d-1, 43 to 8 µmol PIP m-2 d-1, and 22 to 11 µmol POP m-2 d-1. Significant flux relationships (p < 0.001) were found between POP and particulate organic carbon (POC) and particulate organic nitrogen (PON). The lack of a relationship between POC and PIP fluxes and the large fraction of TPP associated with the PIP pool in both oxic and anoxic traps suggests that future analyses must separate PIP and POP when evaluating biological relationships between C, N, and P. The strong relationships between POC, PON, and POP also suggest that POP is not preferentially remineralized relative to PON and POC with increasing depth in this anoxic environment. P composition was also determined using solid state 31P nuclear magnetic resonance (NMR), and it was found that phosphonates, chemically and thermally inert compounds, are a significant fraction of the TPP pool. Furthermore, these compounds were preferentially removed relative to more bioavailable P esters during a low flux event. Their selective removal suggests that these compounds may be an unrecognized source of bioavailable P under anoxic conditions. [ABSTRACT FROM AUTHOR]