or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The approval portionthe ofwith any articlepermitted only photocopy by is of machine, reposting, this means or collective or other redistirbution This article has This been published in MOUNTAINS IN THE SEA Oceanography BY AnDREW T. FISHER AND C. GEOffrEY WHEAT , Volume 23, Number 1, a quarterly journal of The 23, Number 1, a quarterly , Volume Seamounts as Conduits O for Massive Fluid, Heat, and ceanography S Solute Fluxes on Ridge Flanks ociety. © 2010 by The 2010 by O ceanography ceanography O ceanography ceanography S ociety. ociety. A ll rights reserved. Permission is granted to copy this article for use in teaching and research. Republication, systemmatic reproduction, reproduction, systemmatic Republication, article for use and research. this copy in teaching to granted ll rights reserved. is Permission S ociety. ociety. S end all correspondence to: [email protected] or Th e [email protected] to: correspondence all end O ceanography ceanography S ociety, P ociety, O Box 1931, Rockville, M NW Eifuku Volcano, Champagne vent. The white-smoker chimneys are ~ 20-cm across and ~ 50-cm high, and venting fluids measure 103°C.W hite flocculent mats and elemental sulfur coatings surround the chimneys, and D liquid CO2 droplets rise from the seafloor.From Embley et al., 2007 20849-1931, USA . 74 Oceanography Vol.23, No.1 AbSTRACT. Seamounts play a fundamental role in facilitating the exchange of order of 10 TW, ~ 25% of Earth’s total fluids, heat, and solutes between the oceanic lithosphere and the overlying ocean. geothermal heat output, and ~ 30% of Global heat flow compilations indicate that much of the seafloor loses a significant the oceanic lithospheric heat output fraction of lithospheric heat because of fluid flow from the crust, and most of this (Figure 1A). Only a small fraction of advective heat loss occurs on ridge flanks, areas far from the thermal influence of this advective heat output occurs at high magmatic emplacement at seafloor spreading centers. The driving forces available temperatures at mid-ocean ridges; the to move fluid between the crust and ocean are modest, and most of the seafloor is vast majority occurs at lower tempera- blanketed by low-permeability sediment that prevents vertical fluid flow at thermally tures (generally 5–20°C) on ridge significant rates. Thus, most of the thermally important fluid exchange between flanks, suggesting an associated fluid the crust and ocean must occur where volcanic rocks are exposed at the seafloor; discharge of ~ 1016 kg yr-1 (Figure 1B) little fluid exchange on ridge flanks occurs through seafloor sediments overlying (C. Stein et al., 1995; Mottl, 2003; Wheat volcanic crustal rocks. Seamounts and other basement outcrops focus ridge-flank et al., 2003). This low-temperature flow hydrothermal exchange between the crust and the ocean. We describe the driving rivals the discharge of all rivers to the forces responsible for hydrothermal flows on ridge flanks, and the impacts that these ocean (4 x 1016 kg yr-1), and is about systems have on crustal heat loss, fluid composition, and subseafloor microbiology. three orders of magnitude greater than We show data collected from two ridge-flank areas that illustrate how the extent of the sum of high-temperature hydro- fluid exchange, lithospheric heat loss, and chemical reaction and transport depend thermal discharges at mid-ocean ridges on the rate of fluid flow, fluid residence time, and temperature in crustal hydrologic (~ 1013 kg yr -1). Resulting ridge-flank systems. Seamounts are ideal places to sample crustal fluids as they exit the crust and chemical fluxes impact biogeochemical enter the ocean, to determine their chemical and microbial characteristics, and to cycles for numerous solutes (e.g., Wheat assess the importance of this global hydrogeologic system on the evolution of Earth’s et al., 2003), and may help to sustain lithosphere, ocean, and biosphere. vast subseafloor microbial ecosystems (Edwards et al., 2005; Huber et al., 2006). InTRODUCTION Measurements of conductive heat Unfortunately, there are few direct Oceanic heat flow is generally highest flow typically fall below predictions of measurements of fluid fluxes and chem- close to mid-ocean ridge spreading lithospheric cooling models until an ical compositions from typical ridge- centers and decreases as the lithosphere average lithospheric age of ~ 65 Ma. flank hydrothermal systems. Instead, ages. This broad pattern is a natural This deviation from model predictions, local and regional fluid and heat flows consequence of lithospheric cooling, as and the variability of seafloor heat flow on ridge flanks have been calculated predicted by conductive models based commonly observed on ridge flanks, is from seafloor heat flow deficits, and fluid on plate thickness, basal temperature, generally attributed to hydrothermal composition has been inferred from pore thermal conductivity, and heat capacity circulation. This circulation also fluid samples squeezed from sediments (e.g., Davis and Lister, 1974; Parsons and contributes to geochemical reactions collected just above the basalt contact Sclater, 1977). Seafloor heat flow is also that facilitate the exchange of solutes (e.g., Elderfield et al., 1999; Wheat highly variable near spreading centers, between the crust and ocean, redis- and Mottl, 2004). where sediment cover is thin or patchy tribute elements within the crust, and One reason for the lack of direct and volcanic rocks are exposed at the alter the physical state of plates as they measurements and samples from typical seafloor across large areas. Conductive age (e.g., Staudigel et al., 1981; Wilkens ridge-flank hydrothermal systems is seafloor heat flow through ridge flanks, et al., 1991; Alt, 2004). the difficulty in locating sites of low- areas far from the thermal influence Earth’s geothermal heat output is temperature discharge. Vent fields on of magmatic emplacement at seafloor about 44 TW, with most heat loss occur- mid-ocean ridges are often located by spreading centers, tends to be less vari- ring through ocean basins (e.g., Sclater detecting small thermal, chemical, and able, particularly where sediment cover et al., 1980; Pollack et al., 1993). Seafloor particle anomalies tens of meters above is more continuous. hydrothermal heat output is on the the seafloor (e.g., E. Baker and Massoth, Oceanography March 2010 75 12 1987). Low-temperature discharge on Sclater et al. (1980) 10 Mottl (2003) ridge flanks is unlikely to create easily identifiable plumes because differences 8 Stein et al. (1995) in physical and chemical properties 6 between crustal fluids and ocean bottom 4 water can be so small (Wheat et al., Global cumulative 1997). Seamounts and other basement 2 advective heat output (TW) outcrops provide readily identifiable A 0 windows into ridge-flank conditions and River discharge to the ocean processes that are important, but other- T ~ 5 °C mean wise would be difficult to quantify. ridge flank 16 10 In this paper, we describe the critical ~ 20 °C role that seamounts play in global-scale, ridge-flank hydrothermal processes. ~ 65 °C Seamounts were initially defined as 15 10 submarine volcanic constructions rising hydrothermal discharge (kg/yr) Global cumulative B at least 1000 m above the surrounding 0102030405060 Seafloor age (Ma) seafloor (Menard and Ladd, 1963). Some 5 10 Fluid discharge/seamount (kg/yr) of the basaltic edifices discussed in this ) T ~ 5 °C 7 mean 10 paper do not meet this strict defini- 104 Dorado Seamount tion, either because they never were 106 103 1000 m high or because they are now ~ 20 °C 105 mostly buried by marine sediments. To 2 10 ~ 65 °C be consistent with other studies in this 104 1 special issue of Oceanography, we adopt 10 Baby Bare Seamount a more recent definition (Schmidt and 1000 Fluid discharge/seamount (kg/s 0 C 10 Schmincke, 2000; Staudigel and Clague, 104 105 106 Hydrothermally active seamounts on ridge flanks 2010) and refer to all volcanic edifices on the seafloor that were originally ≥ 100 m Figure 1. Calculated ridge-flank hydrothermal advective heat output, fluid discharge, and discharge tall as “seamounts.” per seamount. (A) Global cumulative advective heat output as a function of seafloor age (data from Mottl, 2003, and references therein). Red dotted line = C. Stein et al. (1995). Blue dashed line In the next section, we describe the = Sclater et al. (1980). Green short dashed line = Mottl (2003). Most of this advective heat loss driving force of ridge-flank hydro- occurs on ridge flanks, areas far from the thermal influence of magmatic emplacement at seafloor spreading centers. (B) Global cumulative fluid discharge necessary to advect the amount of heat thermal circulation flowing through estimated by Mottl (2003), plotted as function of age. Curves are shown for assumed temperature seamounts, and discuss the potential for differences between bottom water and hydrothermal fluid of 5°C, 20°C, and 65°C. Lower thermal values are most typical of ridge-flank hydrothermal circulation that mines lithospheric heat. The higher thermal value is characteristic of weak ridge-flank circulation that results in significant local Andrew T. Fisher ([email protected]) is fluid and rock alteration, but has little regional thermal influence. (C) Calculated fluid discharge Professor, Earth and Planetary Sciences per seamount, assuming that all of the fluid flow estimated earlier passes through seamounts. The Department and Institute for Geophysics number of hydrothermally active seamounts is estimated to be somewhere between 104 and 106, based on mapping and seamount population estimates by Wessel (2001) and Hillier and Watts and Planetary Physics, University of (2007), and the observation that, of the seamounts and outcrops that have been surveyed, a signifi- California, Santa Cruz, Santa Cruz, CA, cant fraction appear to be hydrothermally active (Fisher et al., 2003a, 2003b; Hutnak et al., 2008; USA.