Carbonate Chemistry Dynamics of Surface Waters in the Northern Gulf of Mexico

This paper presents the results of two cruises in the Northern Gulf of Mexico in 2008 that investigated local and short-term factors influencing the carbonate chemistry dynamics and saturation state with respect to aragonite (Ω_aragonite) of surface seawater in this region. One cruise covered much of the northern half of the Gulf, and the other focused on the coastal zone west of the Atchafalaya Bay outlet of the Mississippi River—the region where the hypoxic “dead zone” occurs on the Louisiana shelf. Offshore waters (>100 m depth) exhibited only small variations in CO₂ fugacity (fCO₂), total alkalinity (TA) and Ω_aragonite. Values were close to those typically observed in subtropical Atlantic Ocean and Caribbean Sea waters of similar salinity. However, inner shelf waters (<50 m depth) exhibited large variations in fCO₂, TA, and Ω_aragonite that were not directly related to salinity or distance from the Mississippi River plume. Changes in TA values were not the result of simple mixing of end-member freshwater and seawater TA concentrations but exhibited a minimum in values near salinity of 25. This minimum could be the result of microbial recycling across salinity gradients, biological removal of alkalinity by formation of calcium carbonate or mixing of a third end-member with a low alkalinity such as Terrebonne Bay. All waters were supersaturated with respect to aragonite. Offshore waters had an average Ω_aragonite of 3.86 with a standard deviation of only ±0.06 and inner shelf waters had a range in Ω_aragonite values from 3.9 to 9.7 with a median of 4.3. Shelf water Ω_aragonite values were elevated relative to the offshore as a consequence of both high TA input from Mississippi River and biological drawdown of CO₂. A dominant factor controlling Ω_aragonite distribution in offshore waters with relatively constant temperatures was fCO₂, with higher supersaturation occurring in areas with low fCO₂.