Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Sediments were collected and profiled immediately with voltammetric Hg/Au microelectrodes deployed on a computer-controlled micromanipulator (Beckler et al., 2016). Sediment pore waters were then extracted from the same core and either preserved until analysis or analyzed immediately onboard ship. Sampling and analyses were conducted immediately after sediment collection to minimize artifacts from exposure to the atmosphere.

Sediment cores were obtained by a MC-800 multi-corer and profiled within 30 minutes with voltammetric Hg/Au microelectrodes deployed on a computer-controlled micromanipulator. After profiling, sediments were immediately sliced under N₂ atmosphere and pore waters extracted by centrifugation at 3000 rpm under N₂ atmosphere. Finally, pore waters were immediately filtered (0.2 µM PSE Puradisc syringe filters, Whatman) under N₂ atmosphere and either preserved at -20C until analysis (Br⁻, NO₂⁻, NO₃⁻, SO₄²⁻), preserved at 4C after acidification (NH₄⁺), dispensed directly into reagents for analysis (Fe(II), Fed, Mnd, SPO₄³⁻), or analyzed immediately (DIC, TA). Br⁻, NO₂⁻, NO₃⁻, and SO₄²⁻ were measured by non-suppressed HPLC with UV detection (Beckler et al., 2014). NH₄⁺ was measured spectrophopotmetrically by the indophenol blue method (Strickland and Parsons, 1972), Fed and Fe(II) were measured by the ferrozine method after addition or not of hydroxylamine (Stookey, 1970). Mnd was measured by the porphyrin kinetic spectrophotometric method (Madison et al., 2011) modified to account for dissolved Fe(II) interferences (Owings et al., 2020). SPO₄³⁻ 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 was measured by flow injection analysis with conductivity detection after spiking samples with 10 mM ZnCl₂ to prevent dissolved sulfide interferences (Hall and Aller, 1992). Finally, TA was measured by acid titration in an open-cell with continuous pH measurements (Dickson et al., 2007; Rassmann et al., 2016). All errors reported for the electrochemical measurements represent the standard deviation of at least triplicate measurements. Errors of all other parameters represent the analytical error propagated from calibration curves, dilution, and instrumental drift.

Problem report: Some of the data are missing because pore water volumes were too low to make all the measurements.

Chromatographic data were processed using Voltint (Bristow and Taillefert, 2008), a Matlabᵀᴹ-based software developed for these applications. Spectrophotometric measurements were processed manually.

BCO-DMO Processing:
- added date column using original year, month, and day columns.

NSF Award Abstract:
Iron is a limiting nutrient in the world's oceans and plays a key role in regulating the growth of phytoplankton. The main sources of iron to the open ocean are the atmosphere, through wind-blown terrestrial dust, and the seafloor, through input from continental shelf sediments. While atmospheric inputs have been well-studied, the oceanic input of iron from sediments has only sparsely been measured and, as a result, the relative importance of the sediment-derived iron to the iron pool and, ultimately, primary productivity in the oceans is poorly understood. In this study, researchers will examine the chemical properties of sediment-derived iron in the oceans to assess its contribution to the iron used by phytoplankton. Results from this study will further our understanding of iron inputs to the ocean and their importance to ocean primary productivity. The project will contribute to the training of graduate students, as well as provide educational opportunities such as a day at sea for undergraduate students in engineering and physical science.

The atmosphere and continental margin sediments are the main source of the limiting nutrient iron (Fe) to the open ocean. Yet, the chemical form of iron from sediments has not been well examined and only quantified as reduced iron or the dissolved iron passing through 0.45 µm filters. The kinetics of iron oxygenation suggests it should precipitate rapidly in the overlying waters, challenging the view that sediments are important sources of iron for primary production. To establish whether the flux of iron from sediments has important implications for primary productivity, possibly rivaling atmospheric inputs, it is necessary to demonstrate that ferric iron originating in sediments is under the form of stable iron species with potential for a high residence time in the water column. The overall objective of this project is to test the hypotheses that iron fluxing across the sediment-water interface in continental margin sediments is dissolved under the form of organic-Fe(III) complexes and that the magnitude of the iron flux is influenced by the redox conditions in the overlying waters, the composition of the complexes, and the biogeochemical processes in the underlying sediments. To test these hypotheses, the flux and speciation of dissolved Fe(III) will be quantified in the sediments of the Carolina depocenter and the Gulf of Mexico, and the biogeochemical processes regulating the production and the flux of iron as a function of the redox regime of the environment will be determined using in situ measurements and state-of-the-art voltammetric and chromatographic techniques.