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Hydrothermal circulation is one of the primary ways the solid earth interacts with the ocean/atmosphere system and is a consequence of plate tectonics. Early in the development of plate tectonics, the large difference between the observed heat flow and the higher heat flow predicted from cooling lithospheric models was recognized to be a consequence of heat transfer by water flow in the crust. The required heat transfer implies that the equivalent of the volume of the whole ocean cycles through the oceanic crust every few million years. The interaction between the sea water and the oceanic crust significantly affects the chemistry of the oceans and the atmosphere.
Figure 1: Schematic illustration of estimation of hydrothermal flux from the heat flow anomaly. The anomaly, or difference between the heat flow predicted by a lithospheric cooling model and that observed (shaded) is presumably transported by water flux. The flux is thought to be divided into a near-ridge, high temperature, "active" flow and an off-ridge, low temperature, "passive" flow. At a sealing age, the observed and predicted heat flow approximately coincide. The data can be presented either in raw form (top) or as the fraction of the predicted heat flow that is observed (bottom).
I have used the global heat flow data set to better constrain the amount of hydrothermal heat loss and water circulation, and find several important results (Stein and Stein, 1992; Stein et al., 1995).
Figure 2: Cumulative predicted, observed, and inferred hydrothermal heat fluxes as a function of age. The lines connect the points whose values were computed. For clarity, the 1 Ma point is not plotted, and the observed values are offset. Error bars are one standard deviation of the data. Bottom: Cumulative inferred hydrothermal heat flux for 0-65 Ma.
First, about 2/3 of the hydrothermal heat loss occurs by off-ridge and presumably low-temperature flow in crust older than 1 Myr. Because the water temperature for near-ridge flow exceeds that for off-ridge flow, the near-ridge water flow should be even a smaller fraction of the total water flow. Hence in estimating fluxes from geochemical data, use of the high water temperatures appropriate for the ridge axis may significantly overestimate the heat flux for an assumed water flux, or underestimate the water flux for an assumed heat flux. Second, the ratio of observed to predicted heat flow increases with age until about 65 Myr, but shows little dependence on the sediment thickness at the sites. In particular, contrary to earlier suggestions, about 100-200 m of sediment is generally neither necessary nor sufficient to bring heat flow to the value predicted by lithospheric cooling models. The most straightforward inference from these observations is that the fraction of heat transported by hydrothermal flow varies primarily with crustal age and at most secondarily with the sediment thickness. Third, although heat flow in young lithosphere is highly variable, due presumably to local hydrologic complexities, the average heat flow decreases approximately linearly from near the axis to about 20 Myr and then is roughly constant to about 50 Myr.
Recently Hofmeister and Criss (2005) have suggested that the total global heat flow is about 30 TW, about 25% less than previously estimated by Pollack et al. (1993). The main difference between the two estimates is whether oceanic heat flow values are based on the predicted from conductive lithospheric cooling models or the lower measured values, reflecting the additional near-surface heat transfer by hydrothermal circulation. While hydrothermal circulation is spectacularly displayed at hot springs at midocean ridges (see photographs at the top of this web page) detailed heat flow measurements and pore water chemistry do show that significant amounts of water flow near the top of the basaltic oceanic crust and transfer heat within off-axis regions.
Often water is discharged at basement highs and outcrops associated with high heat flow. However, the lower surrounding areas are ofen associated with significantly lower heat heat flow where heat has been removed by hydrothermal circulation. Since most heat flow measurements in younger crust are made in basement lows with sufficient sediment for the heat flow equipment to penetrate, measured heat flow is biased towards lower values and underpredicts the total heat flow. For more detailed discussion see Pollack et al. (2005).
This effect of high heat flow on basement highs due to water discharge and lower heat flow due to removal of heat by hydrothermal circulation in the surrounding lower region is discussed in Stein and Stein (1997). Detailed heat flow measurements made near basement highs are shown below.
Figure 3: Smoothed surface plot of heat flow data for the eastern portion of the FlankFlux area (top) and Galapagos Spreading Center (bottom). Data are shown as heat flow fraction, the ratio of the observed value to that predicted by a model without hydrothermal cooling. Except near basement highs, most measurements have heat flow fractions less than one, indicating significant lateral heat transport by hydrothermal flow. Heat flow data from Davis et al. (1992) and Green et al. (1981).
References
References
Davis, E., D. Chapman, M. Mottl, W. Bentkowski, K. Dadey, C. Forster, R. Harris, S. Nagihara, K. Rohr, G. Wheat, and M. Whiticar, FlankFlux: nature of hydrothermal circulation in young oceanic crust, Can. J. Earth Sci., 29, 925-952, 1992.
Green, K. E., R. P. Von Herzen, and D. L. Williams, The Galapagos Spreading Center at 86 degrees W: A detailed geothermal field study, J. Geophys. Res., 86, 979-986, 1981.
Hofmeister, A. M., and R. E. Criss, Earth's heat flux revised and linked to chemistry, Tectonophysics, 395, 159-177, 2005.
Pollack, H. N., S. J. Hurter, and J. R. Johnston, Heat loss from the earth's interior: analysis of the global data set, Rev. Geophys., 31, 267-280, 1993.
Stein, C. A., and S. Stein, Constraints on hydrothermal heat flux through the oceanic lithosphere from global heat flow, J. Geophys. Res., 99, 3081-3095, 1994. For pdf click here
Stein, C., and S. Stein, Estimation of lateral hydrothermal flow distance from spatial variations in oceanic heat flow, Geophys. Res. Lett., 24, 2323-2326, 1997. For pdf click here
Stein, C., S. Stein, and A. Pelayo, Heat flow and hydrothermal circulation, in: Physical, chemical, biological and geological interactions within hydrothermal systems, AGU Mono., edited by Humphris, S., L. Mullineaux, R. Zierenberg and R. Thomson, Am. Geophys. Un., Washington, D.C., 425-445, 1995. For pdf click here
Von Herzen, D. P., E. E. Davis, A. Fisher, C. A. Stein, and H. N. Pollack, Comments on "Earth's heat flux revised and linked to chemistry" by A. M. Hofmeister and R. E. Criss, Tectonophysics, 409, 193-198, 2005. For pdf click here
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