CO2SYS is an excellent tool for estimation of aragonite saturation in seawater. Here is a link:

http://cdiac.ornl.gov/oceans/co2rprt.html

Here is another link to an Excel version that I developed (12th bullet on this page):

http://www.ecy.wa.gov/programs/eap/models.html

Thanks Greg .

I use CO2SYS as well. Keep in mind that you need at least 2 chemical parameters (pH, total alkalinity, DIC), temperature, salinity, and chl-a (if possible, but not critical). The more variables, the better.

Thanks Jeff I have pH and total alkalinity, temperature, salinity data. I will download CO2SYS and try using it. Will update you guys on outcome.