Dataset: Benthic chamber geochemical data obtained with an in situ benthic lander from the R/V Savannah at sampling stations across the Louisiana shelf and slope in the Northern Gulf of Mexico during four cruise between July 2021 and July 2022

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.