CO2-depleted fluids from mid-ocean ridge-flank hydrothermal springs

Abstract-Hydrothermal spring fluids were collected from Baby Bare, a basement outcrop on the sediment-covered eastern flank of the Juan de Fuca Ridge overlying 3.5 Ma-old crust. These waters are venting at 25°C but have cooled from 64°C within the upper 200-300 m of the upflow zone during their ascent. Unlike ridge-axis hydrothermal vent fluids previously sampled, the total CO2 content of the endmember spring fluids (0.85 Mmol kg-I) is depleted with respect to bottom seawater. The very low alkalinities (0.43 rneq L-I) and high Ca2+ concentrations (55 mmol kg-I) of endmember spring fluids suggest that the removal of C in the spring fluids is associated with carbonate precipitation in the igneous basement. Assuming that 8-20 % of the total ridge-flank heat loss rate of 50 X 10 18 cal yr- I (6.6 TW) is removed by porewater advecting from the sediment at 2:25°C (an upper limit dictated by the global input rate of Mg+2 to the oceans; Mottl and Wheat, 1994), a maximum global carbon sink of 1.0-2.6 X lOll mol yr- I ( 1.2-3.1 Mton-C yr- I) and a maximum global alkalinity sink of 140-340 X 109 eq yr-I are calculated for warm (2:25°C) ridge-flank hydrothermal circulation. This carbon flux is only 2-5 % of current estimates of subaerial and submarine volcanic CO2 emissions (-50 X 1011 mol yr- I), indicating that hydrothermal alteration of oceanic crust on young mid-ocean ridge flanks at temperatures of a few tens of degrees Celsius is only a relatively minor sink for carbon on a global basis. It is still possible, however, that ridge-flank alteration at lower temperatures «25°C) is an important component of the global carbon budget. Copyright © 1998 Elsevier Science Lt