The remarkable longevity of submarine plumes: Implications for the hydrothermal input of iron to the deep-ocean

doi:10.1016/j.epsl.2013.09.008

Earth and Planetary Science Letters

Volume 382, 15 November 2013, Pages 66-76

https://doi.org/10.1016/j.epsl.2013.09.008 Get rights and content

Highlights

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We present laboratory experiments simulating hydrothermal submarine plumes.
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Sedimentation from submarine clouds is driven by particle diffusive convection.
•
This process can extend the longevity of event plumes to 2 yr.
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Complete dissolution of fine sulfide minerals occurs prior to their sedimentation.
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Hydrothermal clouds could provide 75% of the global dissolved iron in the deep-ocean.

Abstract

The longevity of submarine plumes generated at sea-floor hydrothermal systems constrains the hydrothermal input of chemical species into the deep-ocean. Decades of observations of episodic “event plumes” suggest that a key process governing the dynamics of an hydrothermal cloud spreading out laterally from a buoyant rising plume is the production of internal layering. Here, we use analog experiments on turbulent, hot particle-laden plumes and clouds to show that this layering occurs where particle diffusive convection driven by the differential diffusion of heat and small mineral precipitates gives rise to a large scale double diffusive instability. Where hydrothermal clouds are enriched in fine minerals, this “particle diffusive convection” can extend the longevity of an event plume to 2 yr after its emplacement. The very long residence time imposed by diffusive convective effects enables complete dissolution of fine sulfide and sulfate minerals. We develop a new theoretical model that includes both sedimentation and dissolution processes to quantify the potential amount of iron produced by the dissolution of iron-sulfide minerals settling through the cloud by diffusive convection. A key prediction is that the concentration of dissolved iron in hydrothermal clouds can reach up to $19 \pm 3 nM$ , which represents about 5% of the global hydrothermal discharge. If these results are representative of all hydrothermal vent fields, hydrothermal systems could provide 75% of the global budget of dissolved iron in the deep-ocean. Regionally, this flux is expected to scale in magnitude with mid-ocean ridge heat flow, consistent with observations and global ocean models.

Introduction

Sea-floor hydrothermal activity at mid-ocean ridges significantly influences the Earthʼs global energy budget (Elderfield and Schultz, 1996) as well as the trace metal concentration in the oceans (Bruland and Lohan, 2004, German and Von Damm, 2004, Jenkins, 2004, Ramondenc et al., 2006). Among the variety of manifestations of this process, high-temperature turbulent plumes erupting from black smoker chimneys are important conduits for the transport and dispersion of particulate material within the deep-ocean. These complex multiphase flows propel micron-sized metal-rich sulfides, sulfates and oxyhydroxides (Feely et al., 1987) to several tens to hundreds of meters above the sea floor (Speer and Rona, 1989), where they persist and dissolve to varying degrees. The turbulent entrainment and mixing of ocean water into these plumes can enhance precipitation of dissolved chemical species such as iron and manganese (German et al., 1991). Real-time observations suggest that most of these suspended mineral grains are carried upward to the neutral buoyant depth for the particle–fluid mixture where the flow spreads out laterally. This material is, in turn, advected potentially to great distances from the source before either settling to the sea floor or dissolving into the water column (Feely et al., 1992, Elderfield and Schultz, 1996, Langmuir et al., 1997, German and Von Damm, 2004). Because the residence times of entrained minerals can be very long, understanding the dynamics of sedimentation and dissolution is a central issue to constrain the delivery and distribution of chemical species within the deep-oceans.

Hydrothermal plumes are characterized by at least two styles of emission: “chronic” discharges from sustained seawater–oceanic crust interaction (Massoth et al., 1998), and episodic “event” plume (or megaplume) discharges (Baker et al., 1987, Baker et al., 1989), the origin of which is contentious (e.g., Baker et al., 2011). Event plumes are commonly large axisymmetric structures such as EP86 (Baker et al., 1987) and EP87A (Baker et al., 1989). The recent discovery of event plumes associated with a 2008 eruption (EP08A-H) on the Northeast Lau Spreading Center (NELSC) reveals that megaplumes can also be characterized by a thin-layered structure (Baker et al., 2011). Although layering is particularly pronounced in EP08A-H, this enigmatic structure has been observed in a number of past event plumes. Striking examples include EP89 at South Juan de Fuca Ridge (Fig. 1A), EP93A and EP93B at North Juan de Fuca Ridge (Baker et al., 1995), EP96A at Gorda Ridge (Fig. 1B), EP01 at Gakkel Ridge (Edmonds et al., 2003), EP03 at Carlsberg Ridge (Murton et al., 2006), and EP08A-H at NELSC (Baker et al., 2011).

The origin of layering and its influence on the dynamics of particle suspension and sedimentation are poorly understood. Previous explanations for layering invoke pulsed eruption activity (Baker et al., 2011). However, this picture is merely intuitive and the regularity of the layer thicknesses in Figs. 1A and 1B would require unrealistically periodic time-dependent variations in the discharge rate at the vent. An alternative mechanism is the process known as “particle diffusive convection”, which leads to periodic layering by definition (Green, 1987, Huppert et al., 1991, Hoyal et al., 1999, Carazzo and Jellinek, 2013). Particle diffusive convection is a class of multicomponent convection (Turner, 1985) driven as a result of the differential diffusion of heat and very fine particles in particle-laden suspensions (Burns and Meiburg, 2012, Yu et al., 2013). This phenomenon is recognized to be important in hydrothermal clouds where concentrations of dissolved salts and precipitates are high and initial temperature gradients are strong (Hoyal et al., 1999) but a detailed analysis of the effects of this process on the dynamics of submarine clouds is currently missing in the literature. Here, we extend Hoyal et al. (1999) and Carazzo and Jellinek (2013) to the context of hydrothermal clouds in order to understand the enigmatic layered structure observed in event plumes.

We present a series of new laboratory experiments simulating hot particle-laden plumes capable of forming stable neutrally buoyant clouds. Dynamic scaling laws are used to test the reliability of our experiments in simulating hydrothermal plumes. Our laboratory experiments show that the neutrally buoyant clouds resulting from the rise and spread of turbulent plumes may break-up in a series of relatively thin layers due to diffusive convective effects. We show that particle diffusive convection can arise in any hydrothermal cloud and is favored where the mixture is enriched in fine minerals. A sedimentation model validated by our laboratory experiments is used to calculate the residence time of particles in event plumes and to understand how particle sedimentation driven by diffusive convective effects preserve or alter the delivery of iron-particulates in the deep-ocean. Lastly, we discuss the implications of these results on the global mass balance of iron the deep-ocean.

Section snippets

Experimental device

The laboratory experiments consist in injecting upwards a mixture of hot particle-laden water into a salt water-filled tank that is $0.8 m$ high and $1 m \times 1 m$ cross-section (Fig. 2). Prior to an experiment, a well-stirred mixture of either fresh or salt water and well-sorted Custer feldspar particles ( $d_{p} = 300 μm$ , $ρ_{p} = 2600 kg m^{- 3}$ ) is heated in a reservoir disconnected from the tank. In the meantime, the tank is filled with an aqueous NaCl solution with either a stepwise or linear density

Theory

The dynamics of the cloud spreading as well as the main processes involved in the production of layering in our experiments are detailed in Carazzo and Jellinek (2013). Layers form where extensive mixing across the LNB causes a progressive growth of the cloud with an internal approximately linear density gradient defined by variations in temperature and salt concentration. Following cessation of the plume, convection within the cloud driven by the settling of particles through this density

Implications for submarine plumes

The results presented above show that our experimental clouds are unstable to diffusive convective effects. The scaling analysis presented in Section 2.2 suggests that our laboratory-scale plumes are a reliable analog for the transport and sedimentation of fine hydrothermal particles in the deep-ocean. Thus, we now propose to study the influence of this phenomenon on the dynamics of the large-scale flows that are submarine clouds emitted by sea-floor hydrothermal systems. To build this

Discussion

Our results show that the thin layered structure observed in a number of submarine clouds (Fig. 1) forms where conditions for the onset of particle diffusive convection are met. Our simple sedimentation model is independently validated by laboratory experiments and can thus be used to predict the longevity of hydrothermal clouds. A key result is that diffusive convection enables the dissolution of relatively small iron-sulfide particles (Fig. 7, Fig. 8), which provide a substantial amount of

Conclusion

We use an extensive series of laboratory experiments simulating a submarine plume and a thorough review of published data to show that particle sedimentation from hydrothermal clouds can be strongly affected by particle diffusive convection. This process is characterized by a layered structure and enhances the longevity of the cloud. We show that particle diffusive convection is a common phenomenon in hydrothermal clouds and that it governs their dynamics and longevity. On average, event plumes

Notation

c
concentration of solute in solvent
$d_{p}$
particle diameter (m)
g
acceleration of gravity (m s⁻²)
$g^{'}$
reduced gravity (m s⁻²)
h
layer thickness (m)
m
mass of dissolving substance (kg)
t
time (s)
y
particle mass fraction
z
vertical distance (m)
$z_{m}$
plume maximum height (m)
A
area available for dissolution (m²)
C
particle concentration
F
buoyancy flux (m⁴ s⁻³)
$F_{D}$
diffusive flux of particles per unit of area (kg m⁻² s⁻¹)
$F_{I}$
settling flux of particles per unit of area (kg m⁻² s⁻¹)
K
total mass transfer coefficient (kg m⁻² s⁻¹)
N
stratification parameter (s

Acknowledgments

The paper benefited from thorough comments by two anonymous reviewers. The authors thank J. Unger for technical assistance with the experimental device. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Pacific Institute for the Mathematical Sciences (PIMS), and the Canadian Institute for Advanced Research (CIFAR).

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The remarkable longevity of submarine plumes: Implications for the hydrothermal input of iron to the deep-ocean

Highlights

Abstract

Introduction

Section snippets

Experimental device

Theory

Implications for submarine plumes

Discussion

Conclusion

Notation

Acknowledgments

C. R. Géosci.

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In situ observations of the onset of hydrothermal discharge during the 1998 submarine eruption of Axial Volcano, Juan de Fuca Ridge

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Hydrothermal venting in magma deserts: The ultraslow-spreading Gakkel and Southwest Indian Ridges

Geochem. Geophys. Geosyst.

Unique event plumes from a 2008 eruption on the Northeast Lau Spreading Center

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The biogeochemical cycle of iron in the ocean

Nat. Geosci.

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Sediment-laden fresh water above salt water: linear stability analysis

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Particle sedimentation and diffusive convection in volcanic ash-clouds

J. Geophys. Res., Solid Earth

On the rise of turbulent plumes: Quantitative effects of variable entrainment for submarine hydrothermal vents, terrestrial and extra terrestrial explosive volcanism

J. Geophys. Res.

Rift valley/no rift valley transitional mid-ocean ridges

J. Geophys. Res.

In situ chemical mapping of dissolved iron and manganese in hydrothermal plumes

Nature