Table of Contents

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

Samples were collected and analyzed as described in Bolster et al. Briefly, samples were collected from a trace-metal clean rosette, sampled under filtered nitrogen pressure through acid-washed 0.2 μm polyethersulfone filters into acid-washed low-density polyethylene bottles. Iron(II) was measured using luminol chemiluminescence, using diethylenetriaminepentaacetic acid as a masking ligand to correct for interferents (Bolster et al., 2018). Total dissolved metal samples were acidified with Optima grade hydrochloric acid (final concentration 20 mM) and stored at room temperature for several months. Dissolved iron and manganese were determined using inductively coupled plasma mass spectrometry with an offline preconcentration system. 

Additional hydrographic and nutrient data is provided for some samples. Temperature, salinity, and oxygen were measured using a CTD, and measurements of phosphate, nitrate, and nitrite were performed using a nutrient auto-analyzer onboard, as described in Selden et al.

Trace metal samples were collected in 5 L Teflon-coated external spring “Niskin-type” bottles (Ocean Test Equipment) mounted on a powder-coated trace metal clean rosette (Sea-Bird Electronics). Samples were preconcentrated using a seaFAST system (ESI), and quantified using isotope dilution for iron and standard additions for manganese. Analyses were performed on an inductively coupled mass spectrometer in medium resolution mode (Thermo Element 2), using indium as an internal standard. Trace metal quantification was performed using Microsoft Excel 2016.

Temperature, salinity, and oxygen were determined from a Sea-Bird SBE 11plus CTD and a model 43 dissolved oxygen sensor, attached to a sampling rosette for dissolved nutrient concentrations. Nutrients were measured using an Astoria‐Pacific nutrient autoanalyzer.

A number of samples have no measured values for total dissolved iron and manganese, either because sample collection was not logistically possible, or because some of the samples were lost in transit.

BCO-DMO Processing Notes:
- data submitted in csv file "ferrous_iron_seawater_withdates.csv
- added conventional header with dataset name, PI name, version date
- renamed column time_UTC to ISO_DateTime_UTC and formatted as yyyy-mm-ddTHH:MM:SSZ
- sorted records by ISO_DateTime_UTC and depth
- records were sorted by date
- reduced precision of fe, mn, temp, salinity, oxygen, no3, and no2 tp 3 digits

NSF Award Abstract:

The trace metal iron is a key micronutrient throughout the oceans that plays an important role in regulating primary production. Iron (chemical symbol Fe) can exist in different ionization states in the environment, primarily as the reduced form Fe(II) and the oxidized form Fe(III), which impacts its chemical behavior within the water column, and ultimately the ability of researchers to accurately measure its abundance. There is an urgent need in the chemical oceanography community for accurate measurements of Fe(II) in the water column, as currently used methods may overestimate concentrations in water samples collected from certain depths. In this study, researchers from the University of Southern California will significantly redevelop an underutilized methodology to measure Fe(II), and carry out an evaluation and inter-comparison between the two methodologies. This work will enable the refinement of estimates of Fe(II) concentration, present a robust methodology for future use, and clarify conditions where the current methodology can be used effectively. This project will involve strong undergraduate research opportunities, and the research will be conducted in collaboration with an international researcher from India.

Fe(II) is thermodynamically unstable in seawater in the presence of oxygen or nitrate, yet its presence has been reported both in surface waters and in oxygen minimum zones. A rigorous assessment of Fe(II) measurements is important because its chemistry and bioavailability are so different than Fe(III). Fe(II) forms weak complexes, is weakly hydrolyzed and highly soluble in seawater. Fe(III) forms strong complexes, is sparingly soluble and strongly hydrolyzed. The presence of even a modest fraction of Fe as Fe(II) under Fe-limited conditions could increase bioavailability by orders of magnitude. The most widely used methodology to determine Fe(II) at nanomolar to sub-nanomolar levels involves the Fe(II) catalyzed oxidation of luminol, a chemiluminescent (CL) reaction. There are potential artifacts with this methodology, especially in the euphotic zone, where other reduced species might interfere. Researchers will develop a new, modified version of a published alternative method to CL involving the preconcentration of Fe(II) complexed by a synthetic chelator. While this alternative method is prone to artifacts and contamination that has limited its value in the open ocean, new approaches create a plausible path to an accurate and sensitive protocol that is independent of the luminol method. Following the new method development, this project will implement inter-comparisons with the CL approach, which will be conducted in the laboratory, in an anoxic mesocosm system, at a coastal time-series station off the California coast, and in the Eastern Tropical North Pacific.

NSF Award Abstract:
The major process controlling the internal cycling of biologically active trace metals in the oceans is through uptake onto and remineralization from sinking particles. Uptake can occur through active biological uptake into living cells as micronutrients, or chemical adsorption onto sinking materials. This latter process is often referred to as scavenging. The relative importance of these processes is often unclear, especially for elements that are both biologically active and also "particle reactive." The latter characteristic is associated with sparing solubility in seawater and the formation of strong complexes with surface sites, with examples such as iron. Recent evidence suggests that the simplistic view of a sinking particle as a passive surface for metal complexation may require some revision. Investigators James Moffett and Seth John propose to study the chemistry of transition metals within large sinking particles and the resultant effects on metal biogeochemical cycling. They will collaborate with a group at the University of Washington, recently funded to study the microbiology and molecular biology of these particles. The central hypothesis of this project is that reducing microbial microenvironments within large particles support high rates of nitrogen and sulfur cycling, greatly enhancing the particles' influence on metal chemistry. The investigators will study these processes in the Eastern Tropical North Pacific Oxygen Minimum Zone (OMZ). This regime was selected because of the wide range of redox conditions in the water column, and strong preliminary evidence that microenvironments within sinking particles have major biogeochemical impacts.

The primary objective is to investigate the interactions of metals with particles containing microenvironments that are more highly reducing than the surrounding waters. Such microenvironments arise when the prevailing terminal electron acceptor (oxygen, or nitrate in oxygen minimum zones) becomes depleted and alternative terminal electron acceptors are utilized. Within reducing microenvironments metal redox state and coordination chemistry are different from the bulk water column, and these microenvironments may dominate metal particle interactions. For example, reduction of sulfate to sulfide could bind metals that form strong sulfide complexes, such as cadmium and zinc, processes previously thought to be confined to sulfidic environments. Reducing microenvironments may account for the production of reduced species such as iron(II), even when their formation is thermodynamically unfavorable in the bulk water column. Tasks include observational characterization of dissolved and particulate trace metals and stable isotopes in the study area, sampling and in situ manipulation of particles using large-dimension sediment traps, shipboard experimental incubations under a range of redox conditions, and modeling, providing insight from microscopic to global scales. The metal chemistry data will be interpreted within a rich context of complimentary data including rates of nitrogen and sulfur cycling, phylogenetics and proteomic characterization of the concentration of key enzymes. Broader impacts include training of a postdoctoral scientist, international collaborations with Mexican scientists, and involvement of undergraduate students in the research.