The Gulf of Mexico (GOM) has a wide shallow shelf (<20 m deep), occupying approximately 38% of GOM area, whereas the extended continental shelf and continental slope represents 42% of GOM total area (Gore, 1992; Tunnell, 2009) (Fig. 1). The open ocean and abyssal areas (deeper than 3000 m) occupy only 20% of the GOM (Gore, 1992; Darnell and Defenbaugh, 1990; Tunnell 2009; Darnell, 2015). The Gulf of Mexico is semi-enclosed and highly influenced by freshwater input from the Mississippi River. Ocean water enters the GOM through the southern opening, the Yucatan Strait and exits through the Florida Strait (Gore, 1992; Sturges and Lugo-Fernandez, 2005). The Loop Current and Gulf Stream are the main drivers of ocean circulation in the open GOM (Nowlin et al., 2001; DeHaan and Sturges, 2005). High sediment loading, high nutrient concentration, freshwater from the Mississippi River, eutrophication, and seasonal hypoxia events create a unique coastal environment in the Gulf of Mexico, and these events influence the biological and chemical processes further offshore. A spring bloom followed by summer hypoxia is dominant in the GOM continental shelf area (Rabalais and Turner, 2019) which influences the marine ecosystem and biogeochemistry of the GOM. Net primary productivity is influenced by river freshwater input and is generally higher over the shelf (550 g C m^-2 y^-1) than the slope and open water (160 g C m^-2 y^-1) where it becomes oligotrophic (Lohrenz et al., 1999).

Figure 1. Map of the sampling locations in the Gulf of Mexico continental shelf and shelf slope. The “S” stations sampling on a transect south of the mouth of the Atchafalaya River were conducted in August 2011 (circle). The sampling on “T” transect running from the mouth of the Mississippi River to Southwest were conducted in June 2013 (Triangle).

Abstract:

An important pathway for CO₂ sinking in the ocean is via the “biological pump”, driven by the production of particulate organic carbon (POC) in the surface ocean and its subsequent export to the deeper ocean via vertical settling. Here we examine the vertical fluxes of POC in the continental slope of the northern Gulf of Mexico by utilizing the short-lived radionuclide pair ²¹⁰Po – ²¹⁰Pb. The total activities of ²¹⁰Po and ²¹⁰Pb ranged from 2.5 ± 0.35 to 16.52 ± 1.98 dpm 100L^-1 and from 3.47 ± 0.45 to 24.49 ± 3.42 dpm 100L^-1 respectively with the exception of a higher activities of ²¹⁰Po (87.36 ± 11.38 dpm 100L^-1) and ²¹⁰Pb (32.25 ± 4.35 dpm 100L^-1) reported from the near bottom water in the hypersaline anoxic Orca basin. In the upper 150 m of the water column, the inventories of ²¹⁰Pb were consistently greater than those of ²¹⁰Po while the total overall water column inventories of ²¹⁰Po and ²¹⁰Pb were found to be near equilibrium. The ²¹⁰Po fluxes varied between 9.8 ± 6.34 dpm m^-2 d^-1 and 30.7 ± 12.06 dpm m^-2 d^-1 with corresponding POC fluxes between 8.6 mg C m^-2 d^-1 and 37.65 mg C m^-2 d^-1. The fluxes showed a general decreasing trend further offshore, closely following a similar trend in net primary productivity across the slope. The export efficiency ratios “e-ratios” of the biological pump over this slope region was found to vary between 0.027 and 0.11 averaging at 0.05 ± 0.01, which is low. The ²¹⁰Po based and the model-based e-ratio estimates were different however they fall within the range of previous model-based estimates.

Bam, W., and Maiti, K., 2021. ²¹⁰Po-²¹⁰Pb distribution and carbon export in the northern Gulf of Mexico continental slope. Deep Sea Research I, 172, 103535