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We report seasonal water column carbonate chemistry data collected over a three-year period (late 2013 to 2016) at Flower Garden Banks National Marine Sanctuary (FGBNMS) located on the subtropical shelf edge of the northwestern Gulf of Mexico. The FGBNMS hosts the northernmost tropical coral species in the contiguous United States, with over 50% living coral cover. Presented here are results from samples of the upper 25 m of the water column collected from September 2013 to November 2016. Additionally, following a localized mortality event likely associated with major continental flooding in summer 2016, water samples from up to ~250 m depth were collected in the broader FGBNMS area on a rapid response cruise to examine the seawater carbonate system. Both surface (

A simple river-ocean mixing approach has been frequently used to examine estuarine and coastal carbonate system speciation. Coastal areas receiving significant nutrient inputs, however, can have the carbonate chemistry greatly deviated from this mixing-only scheme because of disparate, but spatially coupled biogeochemical processes, i.e., intense primary production in surface waters and elevated respiration in bottom waters; the latter often leads to bottom-water hypoxia (dissolved oxygen or DO concentration − 1 ) and acidification. As a result of land use change, riverine TA input is known to enhance coastal water buffer capacity, although this effect in eutrophic coastal water has not been systematically studied. The physical disturbances of shallow coastal waters by storms can disrupt bottom hypoxia through overturning the water column. This overturn has been proposed to exacerbate bottom water acidification, because of the different exchange rates of oxygen and CO 2 , which could lead to a ‘reset’ of oxygen concentration but little change in the total dissolved inorganic carbon concentration. We used data from the summer 2010 hypoxia cruise in the northern Gulf of Mexico shelf, during which a tropical depression (Bonnie) perturbed the bottom water. Carbonate buffer capacity in both surface and subsurface waters along the salinity gradient suggested that eutrophication-induced surface production and bottom respiration far outweighed the influence of river TA variation and temperature changes in determining carbonate changes on centennial time scales. We propose, based on literature-based CO 2 flux reported in this area, that the benthic (both aerobic and anaerobic) respiration-produced CO 2 flux (with a lesser flux of alkalinity), instead of bottom water reset by storms, could be responsible for further acidifying hypoxic bottom water in addition to water column aerobic respiration.

Surface ocean CO2 fugacity (fCO2) often exhibits large fluctuations and heterogeneity because of multiple controlling factors that pose a challenge for trend analysis, especially in the ocean margins. We propose a new statistical approach, Generalized Additive Mixed Modeling (GAMM), to interpret oceanic fCO2 data in two ocean margins (Japan and Europe) and two open ocean areas. The latter included areas near the Hawaii Ocean Time-series (HOT) and the Bermuda Atlantic Time-series (BATS). This method utilizes day of year, sea surface salinity (SSS), sea surface temperature (SST), and sampling date as predictors. Using this method, we were able to derive multidecadal fCO2 trends with both improved precision and greater robustness to data gaps compared to an existing deseasonalization method used in the open ocean. The fCO2 trend derived by our method for the Japanese margin (1992–2013), the European margin (1989–2014), and the open ocean near HOT (1983–2013) were 2.1 ± 0.6, 1.9 ± 0.7, and 2.0 ± 0.5 μatm year− 1 (mean ± standard deviation of multiple 1° × 1° grids in margins and 5° × 5° grids in open ocean), respectively, and the fCO2 trends were all close to the atmosphere CO2 trend (1.7–1.9 μatm year− 1). Our analysis produced generally smaller standard errors (paired t-test, p ≪ 0.001) than those obtained using the existing method based on the same dataset. In addition, our method was less sensitive to data gaps compared to this existing method. However, for regularly spaced fCO2 times-series data, for example, discrete bottle data collected in the BATS station in 1991–2011, this method was not advantageous over the existing method (1.9 ± 0.2 vs. 2.0 ± 0.2 μatm year− 1). To test the broader applicability of this method, we compared fCO2 trends in the Southern Ocean derived using our method with those from a recently reported Markov Chain Monte Carlo method and found no significant difference between the two sets of values. Therefore, we recommend the application of our method in interpreting fCO2 data in different oceanic environments.

In addition to carbonate dissolution, denitrification represents another pivotal geochemical process that produces alkalinity in the marine environment. Previous studies suggested that such alkalinity can increase seawater buffering capacity and thus enhance atmospheric CO2 uptake when the denitrifying water is exposed to the air in the coastal ocean. In this study, we explored the potential responses of seawater pCO2 to denitrification through three approaches: (1) simulating pCO2 variations in response to various denitrification scenarios, (2) verifying in situ pCO2 data in a well-known denitrification “hotbed”—the Arabian Sea—as well as in anammox-dominated oxygen minimum waters in the Eastern South Pacific, and (3) examining published benthic alkalinity and dissolved inorganic carbon (DIC) flux ratios. In the first approach, we showed that the ratios of alkalinity and DIC addition during denitrification of different model compounds were lower than the slopes of alkalinity and DIC (ΔTA/ΔDIC) along a series of CO2 isopleths corresponding to modern-day xCO2 at different temperatures. In the second approach, we showed that water pCO2 level increased with loss of fixed nitrogen. Last, we showed that benthic alkalinity and DIC flux ratios were also lower than the ΔTA/ΔDIC values derived from the above mentioned CO2 isopleths. Overall, these independent approaches support the conclusion that denitrification-generated alkalinity (together with other alkalinity-altering anaerobic respiration pathways) may not be a notable driving force for enhancing atmospheric CO2 uptake, and concurrent DIC production during denitrification has to be taken into account when discussing changes in seawater buffering capacity along with alkalinity production.

Two brine-charged sites (GB425 and GC233) at the northern Gulf of Mexico (GOM) continental slope were studied for their pore-water advective flow rates and stable carbon isotope geochemistry. Using chloride as a conservative tracer, a pore-water diffusion–advection model suggests that advection dominates pore-water flow in the upper 17 cm sediments at both sites. Moreover, modeled results indicate that there is a downward pore-water flow in GB425 and an upward one in GC233, presumably caused by nearby brine-seep activities. Profiles of pore-water solutes (dissolved inorganic carbon, or DIC, sulfate, and ammonium) can be largely explained by conservative mixing between bottom water and underlying brines, which contain dissolved species that are produced at or below a subsurface reaction front, where sulfate reduction coupled with organic carbon oxidation occurs. Stable carbon isotope data indicate that organic carbon being remineralized has a similar δ 13 C value (− 40 to − 60‰) as that of thermogenic and biogenic methane found in this area. However, without further evidence, the exact carbon source and reaction pathway remain unclear.