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Fluid Flow Through Seamounts and Implications for Global Mass Fluxes

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Fluid Flow Through Seamounts and Implications for Global Mass Fluxes Fluid ¯ow through seamounts and implications for global mass ¯uxes Robert N. Harris Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84102, USA Andrew T. Fisher Earth Sciences Department, University of California, Santa Cruz, California 95064, USA David S. Chapman Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84102, USA ABSTRACT Seamounts contribute to globally signi®cant hydrothermal ¯uxes, but the dynamics and impacts of ¯uid ¯ow through these features are poorly understood. Numerical models of coupled heat and ¯uid ¯ow illustrate how seamounts induce local convection in the oceanic crust. We consider idealized axisymmetric seamounts and calculate mass and heat ¯uxes by using a coupled heat- and ¯uid-¯ow model. By using P. Wessel's global database of ;15,000 seamounts identi®ed through satellite gravimetry, we estimate that the mass ¯ux associated with seamounts is ;1014 kg/yr, a number comparable to estimated regional mass ¯uxes through mid-ocean ridges and ¯anks. In addition, the seamount-generated advective heat ¯ux may be locally signi®cant well beyond the 65 Ma average age at which advective lithospheric heat loss on ridge ¯anks ends. These ¯ows may be important for facilitating geochemical exchange between the crust and ocean and may affect subsea¯oor microbial ecosystems. Keywords: ¯uid ¯ow, heat ¯ow, seamounts, numerical model, mass ¯ux. INTRODUCTION seamounts on a global basis. Fluid ¯ow through seamounts differs Estimates of sea¯oor heat transfer based on the observed de®cit fundamentally from circulation on ridge ¯anks, thought to be respon- between heat-¯ow determinations and models of conductive lithospher- sible for much of the global marine heat-¯ow anomaly. Ridge-¯ank ic cooling indicate that ¯uid ¯ow is responsible for 34% of the global circulation extracts heat advectively from most of the sea¯oor through oceanic heat ¯ux and is thermally signi®cant, on average, to 65 Ma large-scale lateral ¯ow on average, to a mean crustal age of 65 Ma (Parsons and Sclater, 1977; Stein and Stein, 1994). Processes respon- (e.g., Stein and Stein, 1994), whereas ¯ow through seamounts has sible for limiting advective heat ¯ux between the oceanic crust and the mainly a local to regional in¯uence, and occurs at a temperature close ocean include increasing accumulations of low-permeability sediments to that of bottom water. Seamount circulation may continue within that cap relatively high permeability basement, decreasing basal heat some of the oldest sea¯oor, where it can constitute a signi®cant fraction input available to drive ¯ow, and decreasing crustal permeability with of lithospheric heat loss. Although several recent studies have exam- increasing crustal age (e.g., Anderson and Hobart, 1976; Sclater et al., ined ridge-¯ank discharge through basement highs (e.g., Mottl et al., 1980). 1998; Cowen et al., 2003; Kelley et al., 2001), those systems involve Several factors make seamounts ideally suited to overcome these water-rock reactions at temperatures considerably higher than those ¯ow-limiting processes. (1) Bathymetric relief associated with sea- modeled herein. mounts generates thermal buoyancy forces in excess of those present in ¯at sea¯oor. (2) Seamount edi®ces are constructed mainly of extru- GLOBAL DISTRIBUTION OF SEAMOUNTS sive basalt that is likely to have relatively high permeability. (3) Sea- Wessel (2001) catalogued ;14,700 seamounts between lat. 728N mounts tend to remain relatively sediment free much longer than the and 728S by using the gridded vertical-gravity-gradient ®eld derived surrounding sea¯oor, thereby providing areas of exposed basement from altimetry data, and we use this data set in our calculations. Sea- where ¯uid can exchange with the ocean unencumbered by low- mount heights, h(r), are modeled as axisymmetric Gaussian cones tak- permeability sediments. ing the form (Craig and Sandwell, 1988) Sea¯oor heat-¯ow measurements are highly sensitive to hydro- thermal circulation in the underlying crust, but such measurements are 2r2 h(r) 5 A exp , (1) relatively rare across seamounts because the bare-rock environment is 122R2 inhospitable to instrument penetration and because many earlier studies tried to avoid the confounding in¯uence of hydrothermal circulation where r is the radial coordinate, A is the height of the seamount above on lithospheric heat transport. However, several studies have docu- the surrounding sea¯oor, and R is the characteristic radius at an ele- mented seamounts that are hydrologically active and that are stimulat- vation of 0.6A. Wessel (2001) estimated the height and radius for each ing ¯uid ¯ow in the surrounding oceanic crust (Villinger et al., 2002; seamount from peak amplitudes and distance to the zero crossing in Lister, 1972; Fisher et al., 2003a, 2003b; Harris et al., 2000a, 2000b). the vertical gravity gradient ®eld, respectively. Catalogued seamount In this paper we describe numerical experiments that elucidate dimensions are 2±7.5 km in height and 2±50 km in radius; a typical patterns of local ¯uid ¯ow through seamounts and explore the potential height is 2 km, and radius is 10 km (Fig. 1). Histograms of seamount signi®cance of mass and heat ¯uxes associated with circulation through geometry are long tailed and well characterized by power-law distri- q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; August 2004; v. 32; no. 8; p. 725±728; doi: 10.1130/G20387.1; 4 ®gures; 2 tables; Data Repository item 2004118. 725 Figure 2. Results of coupled heat and ¯uid ¯ow for seamount hav- ing aspect ratio (height/width) of 0.2, typical of global population (see Fig. 1), horizontal and vertical permeability of 10212 and 10213 m2, respectively, and basal heat ¯ow of 60 mW´m22. A: Finite- element mesh used for this set of simulations. Vertical exaggeration is 1.2. B: Fluid-¯ow vectors. Vectors are logarithmically scaled. Vast majority of ¯uid and advective heat ¯ow occurs on edi®ce and im- mediately surrounding area; ¯uxes on seamount ¯anks are much smaller. C: Isotherms with contour interval of 10 8C. D: Sea¯oor heat ¯ow. Dashed line shows conductive heat ¯ow (H.F.) in absence of ¯uid circulation. Solid line shows modeled heat ¯ow. Figure 1. Geometries and global distribution of seamounts (data from Wessel, 2001). A: Histogram of seamount heights. Inset shows results in an order of magnitude higher or lower mass ¯ux, respectively. global distribution of seamounts. B: Histogram of seamount radii. The global seamount population is dominated by features with an as- Inset shows histogram of aspect ratios. C: Histogram of spatial den- sity of seamounts. Number in each column represents number of pect ratio of 0.2 (Fig. 1), but simulated ¯uid velocities are relatively seamounts in that age bin (Wessel, 2001). insensitive to observed seamount aspect ratios. The simulations de- scribed here are based on a height and radius of 2 and 10 km, respec- tively, but we have completed sensitivity analyses using a range of butions, such that the actual number of seamounts may be on the order aspect ratios and permeabilities (Fig. DR11), and results cited herein of 100,000, most being too small to be detected by altimetry data are robust to a reasonable range of parameters. (Wessel, 2001). Seamounts are common features and are found on oce- Fluid circulates vigorously through the seamounts in our simula- anic crust of all ages and in all of the world's oceans (Fig. 1). tions, driven by horizontal temperature and density gradients (Fig. 2). In the absence of hydrothermal convection, isotherms would be warped FLUID-FLOW MODEL upward under the seamount, because the sea¯oor is isothermal (2 8C) We simulate ¯uid ¯ow through seamounts by using a ®nite- and conductive heat ¯ow at the sea¯oor is only slightly reduced across element coupled-heat-and-¯uid-¯ow algorithm (Zyvoloski et al., 1997). the seamount due to bathymetric refraction. However, ¯uid recharge This algorithm solves equations appropriate for porous media that ap- over a broad region on the ¯ank of the seamount (Fig. 2B) depresses proximate fractured rock at a large scale. Fluid properties (density, isotherms, and discharge focused across the top of the edi®ce raises enthalpy, and viscosity) vary with temperature and pressure (Harvey isotherms. The resulting heat-¯ow pro®le exhibits a broad low heat et al., 1997). Seamounts are parameterized in terms of their height and ¯ow on the distal ¯ank of the seamount associated with recharge, a radius (equation 1) by using radial coordinates (Fig. 2). The sea¯oor local positive anomaly at r 5 5 km resulting from locally enhanced is assigned hydrostatic pressure and a constant temperature of 2 8C. discharge, and a broad positive anomaly at the central part of the sea- The bottom and distal boundaries of the model are no ¯ow, and the mount. In these examples, seamount permeability is suf®ciently high mesh extends to a radius of 60 km to avoid edge effects. A constant to allow shallower, cellular convection superimposed on a deeper, wid- basal heat ¯ux is used. The model domain is composed of three units, er ¯ow system. These heat-¯ow patterns are similar to those observed a low-permeability lower basement (10219 m2), a higher-permeability near a seamount on the young ¯ank of the East Paci®c Rise, where upper basement 500 m thick, and a seamount edi®ce. Our models are sediment cover is sparse (Villinger et al., 2002). Simulations based on idealized in geometry and use a uniform permeability for the upper the same crustal properties and heat input, but lacking a seamount basement and edi®ce, as appropriate for an order-of-magnitude edi®ce, resulted in ¯uid circulation, at rates several orders of magnitude analysis.
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