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Hydrothermal venting is an important transfer process of energy and elements between the Earth's solid material and the oceans. Compared to mid-ocean-ridge hydrothermal vent fields, those at intra-oceanic island arcs are typically in shallower water depth and have a more variable geochemical fluid composition. Biologically essential trace elements (such as Fe and Mn) are generally elevated in fluids of both deep and shallow hydrothermal vent fields, while vents at shallower water depth influence the photic zone more directly and thus are potentially more relevant for marine primary productivity. However, fluid flux estimations of island arc hydrothermal systems into the surrounding water column are scarce. This study (I) presents a method based on short-lived radium isotopes to estimate submarine hydrothermal discharge (SHD), (II) applies this method at Brothers volcano in the southern Kermadec arc, located northeast of New Zealand, and (III) gives dissolved Fe, Mn and He isotope flux estimates for the Earth´s longest intra-oceanic island arc, the Kermadec arc. The comparison between measured inert He isotope concentrations in the plume with calculated concentrations based on Ra isotopes matched reasonably well, which supports the use of a Ra-based discharge model. Overall, this study represents a novel approach to assess fluid and thus trace element fluxes from one hydrothermal vent field, which can be applied in future studies on various hydrothermal systems to improve geochemical models of element cycling in the ocean. [ABSTRACT FROM AUTHOR]

The naturally occurring radium (Ra) isotope quartet was measured for the first time at an intraoceanic arc hydrothermal setting along the Kermadec Island Arc (SW Pacific). 224Ra (3.7 d half life) and 223Ra (11.4 d half life) are assumed to be tracers solely originating from hydrothermal discharge and have been used to resolve near-field hydrothermal plume dynamics by applying both advection and eddy diffusion-based dispersion models. Assuming dominant advective processes, we identified different horizontal plume age structures at Brothers volcano, with a slow moving plume inside the caldera (0.060 cm/s) and a faster moving plume outside the caldera (0.323 cm/s), indicating a longer plume residence time within the caldera structure. The decreasing trend in Ra activity also allows the calculation of horizontal eddy diffusion coefficients inside and outside of the caldera, with 0.164 m2/s and 3.545 m2/s for 224Ra, respectively. These trends are in accordance with the radial horizontal eddy diffusion model. Our results reveal that net advection might not necessarily play a major role at Brothers, as radial horizontal eddy diffusion reproduces the observed Ra activity distribution to the same degree. Additional model simulations based on a combination of advection and diffusion, favor slow net advection to explain the Ra activity distribution. Within the rising non-buoyant plume, Ra isotopes suggested constant dilution along the first 170 m of the vertical water column and a vertical transport velocity on the order of 0.156 cm/s for altitudes up to 150 m above the vent and around 0.004 cm/s for altitudes above 150 m. When a common source of diffuse and focused venting is assumed and only small amounts of Ra are added to the ascending plume, due to recoil originating from high water to rock ratios, fluids from diffuse vents show a slower migration rate through the subsurface than fluids from focused vents. Our data imply that the fluids from diffuse vents needed approximately five days longer to percolate through the underlying subsurface to reach the water column plume, compared to the fluids from direct vents which emanate through chimney structures. This study demonstrates the potential of short-lived Ra isotopes as tracers of near-field plume dynamics, which will improve our understanding of hydrothermal vents as sources for various trace elements to the global ocean. • First implementation of Ra isotopes for hydrothermal plume studies on intraoceanic arc environments at the Kermadec Arc • Ra-based plume ages suggest a longer residence time within the Brothers volcano caldera than outside • Vertical and horizontal velocities of the dispersing plume were determined from short-lived Ra isotopes • Due to oscillating currents and time integrated Ra signals, transport can be modeled by eddy diffusion as well as advection • Ra isotopes help to distinguish diffuse flow (indirect fluid pathway, aged fluid) and fast focused venting (young fluid) [ABSTRACT FROM AUTHOR]