Brine Formation and Mobilization in Submarine Hydrothermal Systems: Insights from a Novel Multiphase Hydrothermal Flow M
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Brine Formation and Mobilization in Submarine Hydrothermal Systems: Insights from a Novel Multiphase Hydrothermal Flow Model in the System H2O–NaCl F. Vehling1,3 · J. Hasenclever2 · L. Rüpke1 Received: 8 October 2019 / Accepted: 22 October 2020 © The Author(s) 2020
Abstract Numerical models have become indispensable tools for investigating submarine hydrothermal systems and for relating seafloor observations to physicochemical processes at depth. Particularly useful are multiphase models that account for phase separation phenomena, so that model predictions can be compared to observed variations in vent fluid salinity. Yet, the numerics of multiphase flow remain a challenge. Here we present a novel hydrothermal flow model for the system H2O–NaCl able to resolve multiphase flow over the full range of pressure, temperature, and salinity variations that are relevant to submarine hydrothermal systems. The method is based on a 2-D finite volume scheme that uses a Newton–Raphson algorithm to couple the governing conservation equations and to treat the non-linearity of the fluid properties. The method uses pressure, specific fluid enthalpy, and bulk fluid salt content as primary variables, is not bounded to the Courant time step size, and allows for a direct control of how accurately mass and energy conservation is ensured. In a first application of this new model, we investigate brine formation and mobilization in hydrothermal systems driven by a transient basal temperature boundary condition—analogue to seawater circulation systems found at mid-ocean ridges. We find that basal heating results in the rapid formation of a stable brine layer that thermally insulates the driving heat source. While this brine layer is stable under steady-state conditions, it can be mobilized as a consequence of variations in heat input leading to brine entrainment and the venting of highly saline fluids. Keywords Submarine hydrothermal systems · Numerical modeling · Phase separation · Brine layer · Mid-ocean ridges
* F. Vehling [email protected] 1
GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1‑3, 24148 Kiel, Germany
2
Institute of Geophysics, CEN, Hamburg University, Hamburg, Germany
3
Present Address: Institute of Geoscience, Kiel University, Kiel, Germany
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1 Introduction Hydrothermal venting at the ocean floor is a key mechanism of biogeochemical exchange between the solid earth and the global ocean. Fluid circulation through the young ocean floor sustains unique ecosystems (Boetius 2005), mobilizes metals from the crust to the ocean floor to form volcanogenic massive sulfide deposits (Hannington et al. 2011), and is a source of trace elements and isotopes that are now thought to play an important role in global scale biogeochemical ocean cycles (German et al. 2016). Much has been learned about these systems and their role in the Earth System from direct seafloor observations, water column measurements, ocean drilling, and geophysical imaging (Fornari et al. 201
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