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

Abstract

Abstract-Hydrothermal spring fluids were collected from Baby Bare, a basement outcrop on the sedimentcovered 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 CO 2 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 Ca 2 + 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 10 9 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 CO 2 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.

Microbial diversity within basement fluids of the sediment-buried Juan de Fuca Ridge flank

“…This study is the first to utilize a second-generation CORK-II equipped with a stainless steel fluid delivery line that, after flushing, yielded uncontaminated ocean crustal fluid samples derived directly from the crustal aquifer. The chemical signature of borehole 1301A fluids is consistent with previous results from basement fluids collected from borehole 1026B (Wheat et al, 2000;Cowen et al, 2003) and the Baby Bare ridge flank outcrops Sansone et al, 1998;Wheat et al, 2000). In this case, efforts to ensure sampling of pristine crustal fluids produced cellular abundances from borehole 1301A that are 83-93% less than previous reports from borehole 1026B (Cowen et al, 2003;Huber et al, 2006) and 59-67% less than Baby Bare Seamount-derived crustal fluid .…”

Dissolved organic carbon in ridge-axis and ridge-flank hydrothermal systems

“…The inorganic chemistry of Baby Bare and ODP Hole 1026B fluid appears similar to samples from past years with low alkalinities, low concentrations of iron and hydrogen sulfide, and high ammonia concentrations (Table 2; Sansone et al, 1998;Wheat and Mottl, 2000;. Methane and hydrogen concentrations in the ridge-flank fluids are also low compared to those found in axial systems (Table 2).…”

The role of prokaryotes in subsurface weathering of hydrothermal sediments: A combined geochemical and microbiological investigation

“…Brightness is directly related to average atomic mass and is sensitive to large U enrichments which were confirmed by EDAX analysis. microbial activity at the low pH and low effective electron acceptor (O 2 ) concentrations present deep in the subsurface of the hydrothermal sediments and in the deep ocean crust (Hannington et al, 1990a,b;Sansone et al, 1998). Hence, we believe that it is likely that the dominant role of prokaryotes may be in surface and shallow subsurface weathering of seafloor massive sulphide deposits and ocean crust, in the absence of significant subsurface fluid flow.…”

“…We can also reconstruct the pH of low-temperature hydrothermal fluids from measurements of alkalinity and total CO 2 from gas tight samples (Sansone et al, 1998), which produce a pH of 5.5 for the 64°C Baby Bare ridge flank hydrothermal fluids (using the carbonate equilibria described by Zeebe and Wolf-Gladrow (2001)). …”