CO2-DEPLETED FLUIDS FROM MIDOCEAN RIDGE-FLANK HYDROTHERMAL SPRINGS

Hydrothermal spring fluids were collected from Baby Bare, a basement o utcrop on the sediment-covered eastern flank of the Juan de Fuca Ridge overlying 3.5 Ma-old crust. These waters are venting at 25 degrees C but have cooled from 64 degrees C within the upper 200-300 m of the up flow zone during their ascent. Unlike ridge-axis hydrothermal vent flu ids previously sampled, the total CO2 content of the endmember spring fluids (0.85 mu mol kg(-1)) is depleted with respect to bottom seawate r. The very low alkalinities (0.43 meq L-1) and high Ca2+ concentratio ns (55 mmol kg(-1)) of endmember spring fluids suggest that the remova l 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(-1) (6.6 TW) is removed by porew ater advecting from the sediment at greater than or equal to 25 degree s C (an upper limit dictated by the global input rate of Mg+2 to the o ceans; Mottl and Wheat, 1994), a maximum global carbon sink of 1.0-2.6 X 10(11) mol yr(-1) ( 1.2-3.1 Mton-C yr(-1)) and a maximum global alk alinity sink of 140-340 X 10(9) eq yr(-1) are calculated for warm (gre ater than or equal to 25 degrees C) ridge-flank hydrothermal circulati on. This carbon flux is only 2-5% of current estimates of subaerial an d submarine volcanic CO, emissions (similar to 50 x 10(11) mol yr(-1)) , indicating that hydrothermal alteration of oceanic crust on young mi d-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 st ill possible, however, that ridge-flank alteration at lower temperatur es (<25 degrees C) is an important component of the global carbon budg et. Copyright (C) 1998 Elsevier Science Ltd.