Hydrothermal fluxes

Elderfield, H. and Schultz, A. (1996) Midocean ridge hydrothermal fluxes and the chemical composition of the ocean. Anna. Rev. Earth Planet. Sci. Lett., 24, 191-224. 

Hydrothermal Flux from Back-Arc Basin and Island Arc and Global Geochemical Cycle... 

The above argument indicates that hydrothermal CO2 flux from back-arc basins is similar to or greater than that from midoceanic ridges, and thus the hydrothermal flux from back-arc basins as well as hydrothermal flux from midoceanic ridges have to be taken into account when we calculate global geochemical CO2 flux. 

The As (arsenic) concentration of seawater is controlled by input of rivers, sedimentation on the seafloor, weathering of the seafloor, exchange between atmosphere and seawater, volcanic gas input, and hydrothermal input. Previous studies on the geochemical cycle of As have not taken into account the hydrothermal flux of As. Therefore, hydrothermal flux of As from back-arc, island arc and midoceanic ridges to ocean is considered below. 

Hydrothermal flux island arc-back-arc basin axis of midocean ridge... 

Comparison of back-arc hydrothermal flux with midoceanic ridge hydrothermal flux... 

Elderfield and Schultz (1996) estimated midoceanic ridge hydrothermal fluxes using heat and water fluxes estimated by various data (" He/heat, Mg concentration, Sr isotopes, Li isotopes, Ge/Si ratio). Their estimated fluxes are presented in Fig. 3.7 and Table 3.7. 

Hydrothermal flux from back-arc basins estimated based on the H2O flux which was estimated from oceanic crust production rate and seawater/rock ratio at back-arc... 

Fig. 3.7. Comparison of hydrothermal fluxes and river fluxes (data from Table 3.7) (Elderfield and Schultz, 1996).

Back-arc basin hydrothermal fluxes of CO2, As, and Ba are probably higher than the midoceanic ridge hydrothermal fluxes, although we need more detailed investigation. 

Au, Sb and Hg are more enriched into the ores of back-arc basins compared with midoceanic ridge and thus it is likely that back-arc basin hydrothermal flux for these elements is higher than midoceanic ridge hydrothermal flux. However, the concentrations of these elements in back-arc basin hydrothermal solution have not been analyzed. Thus, we need to accumulate analytical data on the concentration of these elements in back-arc basin hydrothermal solution. Further, H2O flux from back-arc basin has to be estimated based on various methods ( He/He, Mg concentration, Li isotope, Sr isotope, Ge/Si ratio) which was argued for midoceanic ridge hydrothermal system by Elderfield and Schultz (1996), but not for back-arc basin by the previous workers. 

Edmond, J., Measures, C., McDuff, E. et al. (1979) Ridge crest hydrothermal activity and the balances of major and minor elements in the ocean The Galapagos data. Earth Planet. Sci. Lett., 46, 1-18. Elderfield, H. and Schultz, A. (1996) Midocean ridge hydrothermal fluxes and the chemical composition of the ocean. Annu. Rev. Earth Planet Sci. Lett., 24, 191-224. 

Shikazono, N. (1993) Influence of hydrothermal flux on arsenic geochemical balance of seawater. Chikyuka-gakii (Geochemistry), 27. 135-139 (in Japanese). 

Fluxes of volatile elements (CO2, S, As) and other elements (Hg, Mn, Ba) due to hydrothermal activities at back-arc basins were calculated. Probably the hydrothermal flux of minor elements concentrated in Kuroko deposits (Sb, Tl, etc.) is large compared with those from midoceanic ridges. CO2 flux from back-arc basins is estimated to be large compared with that from midoceanic ridges. 

In Chapter 3, hydrothermal and volcanic gas fluxes from submarine back-arc basins and island arc are estimated. These fluxes are compared with midoceanic ridge hydrothermal fluxes. Particularly, hydrothermal flux of CO2 is considered and the influences of this flux on global long-term carbon cycle and climate change in Tertiary-Quaternary ages are discussed in Chapter 4. 

As noted above, it is likely that the calcium input fluxes to the oceans and the outputs fluxes are not always equal. According to Equation (5), this means that the seawater 5 Ca ratio can vary with time. The rapidity with which the seawater 5 Ca can change is dictated by the residence time of calcium in seawater. At present, the residence time is estimated to be about 1 million years (e g., Holland 1978). In the past, the residence time could have been larger or smaller, perhaps by as much as a factor of 5 or even 10, depending on the Ca concentration in seawater (Fig. 12) and the riverine, diagenetic and hydrothermal fluxes of calcium to the oceans. 

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