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...floor spreading became apparent, heat flow data presented a conundrum. It was expected that the oceanic lithosphere should release the most heat near spreading centers where the seafloor is youngest (=-=McKenzie 1967-=-; Lister 1972; Parker & Oldenberg 1973), but the variable heat flow seen at young sites commonly included low heat flow values. The difference between theory and observation was resolved by the realiz...

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...ive heat loss from the ocean crust is 10 TW, 25% of Earth’s current geothermal heat output, and 35% of the heat lost through the seafloor by conduction and advection combined (Sclater et al. 1980; =-=Stein & Stein 1994-=-; Mottl 2003) (Fig. 2B). Of this advective heat loss, 2–3 TW occurs at or near seafloor spreading centers, and the remaining 7–8 TW occurs on ridge flanks, far from the magmatic influence of lithosphe...

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...merous physical and chemical surveys have been completed on 0.7–3.6 Ma seafloor on the eastern flank of the Juan de Fuca Ridge (Davis et al. 1992, 1999; Wheat & Mottl 1994, 2000; Thomson et al. 1995; =-=Elderfield et al. 1999-=-; Fisher et al. 2003a; Hutnak et al. 2006; Wheat et al. 2004). Oceanic basement rocks in this area were formed at the unsedimented Endeavour segment of the Juan de Fuca Ridge, but were subsequently co...

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...tes where seafloor heat flow surveys have been conducted: Guaymas Basin in the Gulf of California, Escanaba Trough on the Gorda Ridge, and Middle Valley at the northern end of the Juan de Fuca Ridge (=-=Lonsdale & Becker 1985-=-; Abbott et al. 1986; Davis & Villinger 1992). All three are sedimented spreading centers, located adjacent to continental areas where thick sediments trap heat within the subsurface, compressing ther...

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... lead to curvature in thermal gradients under conditions of conductive heat flow (Fig. 3F). Vertical analytical models of coupled fluid-heat flow have been modified to account for lateral flow (e.g., =-=Lu & Ge 1996-=-), but this is rare in shallow sediments, because the sedimented seafloor is usually flat or gently sloped and lateral pressure gradients tend to be small. An exception to this latter generalization i...

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...he ocean crust is 10 TW, 25% of Earth’s current geothermal heat output, and 35% of the heat lost through the seafloor by conduction and advection combined (Sclater et al. 1980; Stein & Stein 1994; =-=Mottl 2003-=-) (Fig. 2B). Of this advective heat loss, 2–3 TW occurs at or near seafloor spreading centers, and the remaining 7–8 TW occurs on ridge flanks, far from the magmatic influence of lithospheric creation...

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... main classes of sediment temperature probes used during scientific ocean drilling (Fisher & Becker 1993): push in tools that require a dedicated tool run, independent of coring operations (Fig. 1C) (=-=Uyeda & Horai 1980-=-; Davis et al. 1997a); and tools that are integrated with piston coring operations (Fig. 1D) (Horai & Von 1985; Fisher et al. 2007). These instruments are generally deployed 1–10 m beyond the drilled ...

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...heric), and thermal properties of the sediment and rock ⁄ fluid system. The WMA model can be extended to include a second conductive boundary layer below sediments and above permeable basement (e.g., =-=Rosenberg et al. 2000-=-). The influence of basement relief, local convection, and regional advection may be combined if there is outcrop-tooutcrop circulation (Fig. 3C). Seawater entering the crust, circulating laterally, t...

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...ffshore the Nicoya Peninsula, Costa Rica (Fig. 4B), comprises a northern region formed at the East Pacific Rise (EPR), and a southern region formed at the Cocos-Nazca Spreading Center (CNS) (Fig. 7) (=-=Meschede et al. 1998-=-; Ranero & von Huene 2000; von Huene et al. 2000; Barckhausen et al. 2001). The boundary between EPR- and CNS-generated seafloor is a combination of a triple junction trace and a fracture zone trace, ...

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... than erupting at the seafloor. Upper lithologic ‘basement’ in this setting comprises alternating sills and indurated sediments having bulk permeability much greater than that of overlying sediments (=-=Becker et al. 1994-=-; Fisher et al. 1994; Langseth & Becker 1994). This sediment–sill sequence hosts vigorous hydrothermal circulation that leads to essentially isothermal (280C) conditions in uppermost basement (Davis ...

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...r et al. 1997; Larson et al. 1993). Strings of thermal sensors have been deployed in subseafloor borehole observatories as part of long-term monitoring installations (e.g., Davis & Becker 1994, 2002; =-=Foucher et al. 1997-=-). Tools used during scientific ocean drilling and in boreholes are not discussed in the rest of this review, which focuses on measurements made in shallow sediments. Thermal gradient measurements are...

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...te regionally extensive sediments (Fig. 7). (A) (B) Fig. 7. A. Bathymetric map of field area on 18–24 Ma seafloor of the Cocos Plate, eastern Pacific Ocean (index map shown in Fig. 4B) (modified from =-=Hutnak et al. 2008-=-). Dorado outcrop is a site of hydrothermal discharge, whereas Tengosed Seamount is a site of hydrothermal recharge. Gray line demarcates thermal boundary between cooler and warmer parts of the plate....

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...sing hydrothermal recharge and discharge, and allowing thermal and fluid flow regimes to be characterized through coring, drilling, and heat flow surveys (Davis & Villinger 1992; Gieskes et al. 1982; =-=Kastner 1982-=-; Stein & Fisher 2001; Wheat & Fisher 2007). Studies from these areas provide unique insights on coupled fluid–thermal processes within very young seafloor. Middle Valley (Figs 4A and 5A) is bordered ...

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...10 W m–2 within the Dead Dog AAV has been interpreted to result from formation of a diagenetic and hydrologic cap above a shallow hydrothermal reservoir (Davis & Villinger 1992; Davis & Fisher 1994; =-=Stein et al. 1998-=-; Stein & Fisher 2001). Fluid convects vigorously below the cap, helping to maintain isothermal conditions near 280C at 30 m below the seafloor. There may be additional focusing of conductive heat f...

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...thologic ‘basement’ in this setting comprises alternating sills and indurated sediments having bulk permeability much greater than that of overlying sediments (Becker et al. 1994; Fisher et al. 1994; =-=Langseth & Becker 1994-=-). This sediment–sill sequence hosts vigorous hydrothermal circulation that leads to essentially isothermal (280C) conditions in uppermost basement (Davis & Fisher 1994; Davis & Villinger 1992; Davis...