  Ancient hydrology is recorded by sedimentary rocks on Mars. The most
voluminous sedimentary rocks that formed during Mars' Hesperian period are
sulfate-rich rocks, explored by the $Opportunity$ rover from 2004-2012 and soon
to be investigated by the $Curiosity$ rover at Gale crater. A leading
hypothesis for the origin of these sulfates is that the cations were derived
from evaporation of deep-sourced groundwater, as part of a global circulation
of groundwater. Global groundwater circulation would imply sustained warm
Earthlike conditions on Early Mars. Global circulation of groundwater including
infiltration of water initially in equilibrium with Mars' CO$_2$ atmosphere
implies subsurface formation of carbonate. We find that the CO$_2$
sequestration implied by the global groundwater hypothesis for the origin of
sulfate-rich rocks on Mars is 30-5000 bars if the $Opportunity$ data are
representative of Hesperian sulfate-rich rocks, which is so large that (even
accounting for volcanic outgassing) it would bury the atmosphere. This
disfavors the hypothesis that the cations for Mars' Hesperian sulfates were
derived from upwelling of deep sourced groundwater. If, instead, Hesperian
sulfate-rich rocks are approximated as pure Mg-sulfate (no Fe), then the CO$_2$
sequestration is 0.3-400 bars. The low end of this range is consistent with the
hypothesis that the cations for Mars' Hesperian sulfates were derived from
upwelling of deep sourced groundwater. In both cases, carbon sequestration by
global groundwater circulation actively works to terminate surface
habitability, rather than being a passive marker of warm Earthlike conditions.
$Curiosity$ will soon be in a position to discriminate between these two
hypotheses. Our work links Mars sulfate cation composition, carbon isotopes,
and climate change.
