Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
  • Problem Description
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Benthic chamber samples were collected using two in situ benthic landers configured similarly, one tethered to the surface for shelf measurements (< 100 meters (m)) adapted from a previously developed platform (Tercier-Waeber and Taillefert, 2008; Meiggs et al., 2011), the other is a modified version of the free Benthic Experiment Chamber Instrument (BECI) developed for deep-sea measurements (Jahnke and Christensen, 1989). Both landers are autonomous and programmed before deployment. They carry a single benthic chamber, two sampling racks holding 50-milliliter (ml) polypropylene syringes for the injection of a chemical tracer and the sampling of up to 18 samples (Jahnke and Christensen, 1989). The shelf lander chamber is gently mixed using a SBEST Seabird pump, whereas the deep-sea chamber is mixed mechanically at slow speed (5-10 rpm). The benthic chambers are closed using hydraulically-controlled actuators after a settling period of 20 minutes on the seafloor. A chemical tracer is injected 10 minutes after the lid is closed, and the first sample is collected 2 minutes after tracer injection to determine the volume of the overlying waters in the benthic chamber (Rao and Jahnke, 2004). Samples are then collected with different frequencies depending on the time of deployment (ranging from 60 minutes on the shelf to 115 minutes for deep-sea measurements). Both landers were recovered within an hour after the last benthic chamber sample was collected. The benthic water samples were then filtered within an hour after lander recovery onto 0.22-micrometer (µm) Whatman 25-millimeter (mm) Acrodisc syringe filters (PES membrane) and either preserved until analysis or analyzed immediately onboard ship. Samples were preserved at 4 degrees Celsius (C) after acidification (Ca_d, Mn_d) or addition of HgCl₂ (δ¹³C-DIC, TA), preserved at -20 degrees C (NH₄⁺, NO₃^-, Br^-), or dispensed directly into reagents for analysis (SPO₄^3-). Sealed borosilicate glass vials were used for TA, DIC, and δ¹³C-DIC as recommended (Dickson et al., 2007; Wang et al., 2018).

NH₄⁺ was measured spectrophotometrically by the indophenol blue method (Strickland and Parsons, 1972), SPO₄^3- was measured spectrophotometrically using the molybdate-blue method after natural color correction to avoid interferences from dissolved silica and sulfides (Murphy and Riley, 1962). DIC and TA were measured by acid titration in a closed cell with continuous pH measurements (Dickson et al., 2007; Rassmann et al., 2016), except in July 2022 when DIC and TA were measured by cavity ring-down spectrometry (CRDS, Picarro G2131-i) with an automatic CO₂ extraction system (Apollo SciTech AS-D1) and open-cell potentiometric titrations, respectively (Ferreira et al., 2025). δ¹³C-DIC was measured using an isotope ratio mass spectrometer with a high-performance liquid chromatography preparation module for gas samples (Brandes, 2009; Wang et al., 2018). NO₃^- and Br^- were measured by high-performance liquid chromatography (HPLC) using a matrix elimination method developed for seawater samples without dilution (Beckler et al., 2014). Finally, Ca_d and Mn_d were measured by ICP-MS with collision cell to prevent argon interferences.

Internal standards were used to correct for the drift of the instrument, and quality control blanks, standard checks, and certified seawater references were run several times during each run to control accuracy and reproducibility. All calibrations were conducted with at least five standards prepared in a 0.54 M NaCl matrix before each series of measurements. Blanks and quality control checks were run routinely during each analysis. Finally, calibration sensitivities were compared routinely to ensure the accuracy of the methods. For DIC analyses, Dickson DIC-certified seawater samples were run routinely during analyses to validate the accuracy of the method. Errors of all reported concentrations represent the analytical error propagated from calibration curves, dilution, and instrumental drift.

Spectrophotometric measurements were recorded on paper forms specifically designed for each analysis and digitized to process the data. DIC, TA, δ¹³C-DIC, HPLC, and ICP-MS data were acquired by computers. DIC and TA concentrations were determined by non-linear least squares fitting of the DIC and total alkalinity equations in seawater to the titration data (Magette et al., 2025). NO₃^-, and Br^- chromatographic data were integrated using a Matlab-based software developed in-house (Bristow and Taillefert, 2008). TA, δ¹³C-DIC, and ICP-MS data were processed through Excel macro spreadsheet routines developed for each of these analyses.

- Imported the four original files into the BCO-DMO system (original file names: GOM_Summer2021_BenthicChamberData_BCO-DMO.xlsx, GOM_Fall2021_BenthicChamberData_BCO-DMO.xlsx, GOM_Spring2022_BenthicChamberData_BCO-DMO.xlsx, GOM_Summer2022_BenthicChamberData_BCO-DMO.xlsx)
- Flagged "nd", "#N/A", and "--" as missing data values; missing data are empty/blank in the final CSV file.
- Concatenated the four data files into a single data file.
- Added column for the cruise ID.
- Created Date field in YYYY-MM-DD format.
- Renamed fields to comply with BCO-DMO naming conventions.
- Saved the final file as "959033_v1_nGoM_benthic_chamber_2021-2022.csv".

NSF Award Abstract
Ocean acidification is the process that lowers the pH of the ocean over time due to uptake of atmospheric carbon dioxide. This project investigates how chemical reactions in marine sediments exposed to high riverine sediment loads influence ocean acidification in coastal waters. Although we know that ocean acidification affects marine life and commercial fisheries in coastal waters, little is known about how acidification processes in the water column are influenced by reactions occurring in sediments on the sea floor. The role of large sediment deposits from rivers in these processes has also never been investigated. This study will be conducted in the Mississippi River and Gulf of Mexico. The Mississippi River transports a high sediment load to the continental shelf in the Gulf of Mexico and plays an important role in the economy of the southern coast of the United States. Results from this study will be useful to the oceanographic community for increasing understanding of ocean acidification processes in delta and shelf environments. It will also benefit decision makers interested in predicting the role of sediments on the nutrient -rich Louisiana shelf for discharge control purposes. This project also has an important educational component by training undergraduate, graduate, and postdoctoral students, providing experiences at sea for undergraduates, and conducting outreach activities with K-12 students.

The geochemical and microbiological processes responsible for the transformation of particles deposited on the seafloor will be characterized near the Mississippi River mouth and along the nearby continental slope. The release of acids (CO2) and bases (alkalinity) from the sediment will be quantified using autonomous instruments deployed on the seafloor to determine whether sediments contribute to the acidity of the surrounding water column or instead provide bases to buffer the water column from atmospheric CO2 inputs. As the Mississippi River discharge during the later Winter and Spring provides much more sediment to the coastal zone compared to the rest of the year, research cruises will be taken twice a year to determine how seasonal variations in riverine discharge affect the release of acids and bases into the water column. Mathematical models will then be used to predict the effect of seasonal variations on acids or bases release to the water column. This study will therefore provide a quantitative understanding of the role of large sediment depositions to the seafloor on sediment geochemical and microbiological processes and their feedback to the overlying waters. Simultaneously, a large data set will be generated and used to calibrate mathematical models and better characterize benthic-pelagic interactions. Such efforts are needed to predict how continental margins respond to constantly increasing stress from anthropogenic activities.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.