Assessments of Carbonate Chemistry Baseline and Potential Impact of Carbon Dioxide Addition to the Northwestern Gulf of Mexico Shelf Waters

Changing seawater chemistry has a wide range of implications for the marine ecosystem (living resources) and human society. In addition to CO2 uptake from the atmosphere that causes ocean acidification, high levels of CO2 can be found in both low oxygen seawater and anoxic waters due to organic carbon remineralization because marine microbes respire organic matter to generate energy. This type of “metabolic acidification” is particularly notable in areas subject to strong nutrient runoff from large rivers and ocean upwelling. The additive effect of both atmospheric CO2 uptake and internal CO2 generation reduces seawater buffer, pH, and carbonate saturation to the extent equivalent to or worse than predicted by the end of century atmospheric CO2 scenarios. Given the vastness of the global ocean that covers ~70% of the earth's surface, ocean-based carbon dioxide removal (CDR) has been gaining traction in recent years.

The National Academy of Sciences (NAS) released a report examining the feasibility of six approaches for ocean-based CDR, all of which, except the ecosystem recovery method, involve CO2 conversion to either alkalinity or organic carbon (nutrient fertilization, seaweed cultivation, and artificial upwelling and downwelling). The NAS report did not consider direct CO2 injection into the pressurized subsurface ocean, citing societal resistance to the implementation of this approach due to concerns over detrimental effects on deep-sea biota, hence international effort was halted. Nevertheless, a local-scale experiment did reveal interesting behavior of liquified CO2 and the formation of hydrate at various depths of the water column.

The Gulf of Mexico (GOM) is a warm, saline body of water with its top few hundred meters being affected by both the Loop Current that comes through the Yucatan Channel and significant river runoff. The Mississippi-Atchafalaya rivers are the major driving force for biogeochemistry of the northwestern GOM (nwGOM) shelf. Together with wind forcing, both river influence and eddies shed from the Loop Current control the circulation patterns in this shelf region, and the river influence can reach as far south as the U.S.- Mexico border. Other than the extensive river system, smaller rivers in this region also play some role in more localized areas.

In addition, episodic but strong precipitation events introduced by extreme weather events can cause significant continental flooding, which might temporarily overwhelm the coastal region. Due to the potentially strong land-ocean coupling as a result of hydrological continuity as well as shelf circulation that is driven by wind and eddy activities, seawater carbonate chemistry in the nwGOM region can be affected by bottom water hypoxia (i.e., dissolved oxygen less than 2 mg L-1 , Rabalais et al., 2002), and upwelling of deeper water onto the continental shelf is also a likely contributor that may have affected shallow water tropical coral habitat.

Given the potential prospect of industrial scale CO2 sequestration under the seabed on the nwGOM shelf, an assessment of the carbonate chemistry “baseline” in the water column and sediment, as well as potential risks of CO2 leakage on both benthic and pelagic biota, is needed.

This proposed project includes the following tasks:

1. Identify water column datasets in both published and “gray” literature. These datasets include direct carbonate chemistry measurements conducted by both government and academic researchers, and hydrographic measurements that include physical parameters (salinity, temperature, and depth) as well as dissolved oxygen.
2. Use data summarized in Task 1 to generate carbonate chemistry baseline information and its variability for the nwGOM shelf. This will include data from direct measurements and those generated through statistical methods (for example, multilinear regression).
3. Survey the literature for the distribution of heavy metals in the nwGOM shelf sediments, as metal toxicity in contaminated sediments could be enhanced under both natural and artificially induced acidification conditions.
4. Assess hydrodynamic conditions from the near coastal region to the shelf-slope edge and temporal variations.
5. Assess the benthic and pelagic biota distribution in the nwGOM and their seasonal “travel” patterns.
6. Survey the literature for threshold values for carbonate chemistry parameters, for example, critical carbonate saturation state, hypercapnia, and pH levels.