I've only skimmed this article (my copy arrived in today's mail) but I noted this in the summary section:

"The chemically homogeneous bright soils argue for a global acidic ocean, though eolean homogenization of the evaporated salts of a number of acidic bodies cannot be ruled out."

also interesting is:

"High concentrations of phosphorus in a Martian ocean would not be expected if Mars had an active biosphere during the oceans existence."

Greenwood, J.P. and Blake, R.E., 2006, Evidenece for an acidic ocean on Mars from phosphorus geochemistry of Martian soils and rocks, Geology, V.34, No. 11, pp. 953-956.