Introduction

Project Summary

This project is designed to provide the first Gulf-wide distribution of surface ocean pCO₂ and CO₂flux between the surface ocean and the atmosphere and contributes toward the goals of the Ocean Carbon and Climate Change Program (OCCC), the U.S. North American Carbon Program (NACP), and the Ocean Biogeochemistry Program (OCB). The research, while not only offering critical scientific information about the flux of CO₂in the ocean environment, will address the regional variability and dynamics of the atmospheric-ocean system over approximately the last ten years.

Air-sea CO₂ flux in the Gulf of Mexico: What is it and why is it important?

Gulf of Mexico and carbon flux

The Gulf of Mexico (GOM) is a large, semi-enclosed subtropical/tropical sea shared almost equally by the U.S. and Mexico. Fed by more than 150 rivers, the drainage basin of the GOM is comprised of 33 major river systems, and extends over approximately 40% of the landmass. Among the many rivers, the Mississippi and Atchafalaya River System is largest, carrying about 65% of all freshwater to the Gulf. The estuarine system that rims the Gulf also plays a large role. The Gulf of Mexico is distinctive in terms of its river- and estuarine-dominated shelves. River-dominated estuaries characterize the northern Gulf, large marine-dominated bays occur to the east (Pennock et al., 1999), and to the south coastal lagoons dominate. The importance of constraining CO₂ fluxes in the GOM is evident in the modulation and estimation of continental (atmospheric) CO₂ concentrations. Large variability between marine air concentrations impacted by sea-air CO₂ fluxes and continental air masses (see review in Robbins et al. 2009) can introduce significant errors into continental CO₂ flux calculations by atmospheric inversion methods, particularly at regional scales (Chavez et al., 2007, Hales et al. 2008, Lohrenz and Cai, 2006). The drainage basins of the GOM have also experienced great climate and anthropogenic changes (Turner and Rabalais, 2003, Searchinger et al., 2008, Lohrenz et al. 2008), which have influenced and will continue to influence carbon cycle in the GOM (Rabalais et al, 2002, Bianchi and Allison, 2009, Cai and Lohrenz, 2010). Thus the current status of air-sea CO₂ flux represents a big knowledge gap that is critical to our understanding of the carbon cycle and budget in North America and how they may respond to climate and anthropogenic changes in the future.

Figure 1: Cruise tracks of data compilation

Tracks (blue lines) of 196 cruises in the Gulf of Mexico from which pCO₂ data were compiled, as reported at the 2013 Gulf of Mexico Coastal Synthesis Workshop.

Figure 2: Sub-regions of the Gulf of Mexico for this study

1: Open Ocean

2: West Florida Shelf

3: Northern Gulf of Mexico

4: Western Gulf of Mexico

5: Yucatan/Mexico

What is known about the air-sea CO₂ flux in the Gulf of Mexico

The uncertainties in coastal carbon fluxes are such that the net uptake of carbon in the coastal margins remains a poorly constrained term in global budgets (Jahnke, 2010, Cai, 2011). In the GOM, the fluxes of total carbon across key interfaces are poorly constrained and the fate of total carbon in the coastal ocean is poorly understood. Yet, these data are fundamental to understanding the carbon budget.

Since 2007, there has been a rapid increase in coastal ocean CO₂ data being archived in public databases. Under NASA, NOAA and NSF support, data synthesis efforts have also made progress. For the GOM, good progress has recently been made in collating existing in situ CO₂ data, covering the northern GOM around the Mississippi-Atchafalaya River, West Florida shelf, the open ocean area, and offshore Texas (Robbins et al., 2013) (Figure 1). More recent data from several cruises will be added to this compilation as part of this effort.

Based on preliminary data synthesis, the direction and magnitude of the CO₂ flux term in the different sub-regions (Figure 2) can be identified. Once the recent data sets are quality controlled and incorporated into the data base, the data will be parsed out yearly, monthly and seasonally to estimate annual and interannual variability and trends. Large data gaps in specific regions will be addressed through effective interpolation techniques utilizing remotely sensed sea surface temperature and salinity, ocean color, and numerical model output.

With respect to pCO₂, limited previous observational data suggested that during the summer, the Gulf of Mexico is a CO₂ source to the atmosphere with a flux ranging from 0.186 to 3.32 mol/m² /yr¹, while during the winter it is a net sink of -0.22 to -1.18 mol/m²/yr¹ (Coble et al., 2010). In the Gulf of Mexico, (north of 24^oN), the air-sea flux was calculated to be -11.8 Tg C/yr (a net sink). Coastal waters in the northern GOM are a strong CO₂ sink year round and coastal waters of the NE Gulf are a moderate sink. In the late summer, the open waters of the Gulf are a source of CO₂ , but a sink all other times of the year. No or minimal data were available for the western and southern regions of the Gulf.

While there are now six times more data points compared with the dataset on which the last estimate was derived, there are still areas for which researchers have data that were not incorporated. Significant gaps in the data in a number of regions remain. The extra data will help improve temporal coverage to facilitate the study of seasonal variability. The overall trends for the Gulf show near-shore sinks and changes in direction of seasonal air-sea CO₂ fluxes, corroborating with earlier estimates; however, the net uptake is appreciably smaller in the new calculations. Further, trends for specific regions and seasonal and interannual variability can now be explored.

A preliminary annual pCO₂ air-sea flux estimate has been made with the recently acquired dataset (Table 1). The preliminary estimate of the compiled dataset indicates that the Gulf is a sink with a net annual specific flux of -0.19 mol/m²/year, which corresponds to a small uptake for the entire Gulf of -3.57 Tg C/year. This value is smaller than the initial model estimate of -11 Tg C/year with the largest differences apparent in the Northern Gulf. Besides the larger datasets, procedures to calculate the fluxes are improved. In particular, higher-resolution wind speed products and improved gas exchange-wind speed relationships (Wanninkhof et al., 2009) were used in the updated estimates.

Objectives

Our long-term goal is to improve our understanding of the sinks and sources of CO₂ by delineating their distribution through synthesis and modeling efforts.

Our goals are to:

Produce a whole Gulf of Mexico monthly air-sea CO₂ flux climatology with detail in regional and seasonal variations and with an understanding of the associated uncertainties.
Produce a whole Gulf of Mexico inorganic carbon budget based on observational and modeling approach with a sufficient understanding of carbon cycling processes.
Carry out detailed regional studies in data-rich Northern Gulf of Mexico and West Florida shelf with a focus on how seasonal and spatial changes and unusual weather events influence surface water CO₂ distribution and air-sea CO₂ fluxes.
Perform a comparison between model-derived and observation-derived estimates as a means to gain a process understanding of the controls on sea-air fluxes in the different provinces.

To accomplish these goals, we are using the expertise and regional knowledge of team members to compile, coordinate, and synthesize the existing data sets, create a climatology that will allow comparison across years, model the derivative datasets for predictive capabilities, and produce products that will allow the community to continue to explore in depth the carbon budget for the Gulf of Mexico.

Table 1.

Preliminary CO₂ fluxes for the different regions and the total Gulf of Mexico using a computed second moment of monthly wind speeds (<U²>) within gridding of open ocean data into 1° x 1° bins and coastal ocean data into 0.5° x 0.5° bins.

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