Oxygen deficient zones (ODZs) play an important role in the distribution and cycling of trace metals in the ocean, as important sources of metals including Fe and Mn to the ocean, and also as possible sinks of chalcophile elements such as Cd. The Eastern Tropical North Pacific (ETNP) ODZ is one of the three largest ODZs worldwide. Here we present results from two sectional surveys through the ETNP ODZ (23N-14N, 105W-130W) conducted during two cruises of the R/V Roger Revelle, RR1804 and RR1805, from March to April 2018, providing a high-resolution concentrations of dissolved Cd, Ni, Mn, La, Ce, Pr, and Nd in seawater.

Table of Contents

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

Methodology: 

Sampling and analytical procedures:

During RR1804 cruise, regular Niskin bottles (12 L GO-FLO bottles) and Seabird CTD rosette were used to collect seawater samples. Each seawater sample was collected into an acid-cleaned LDPE bottle on board and then filtered in a trace metal clean (TMC) “bubble” on the boat using an AcroPakTM 0.2µm filter and a peristaltic pump and stored in 50 mL low density polyethylene (LDPE) centrifuge tubes.

During RR1805 cruise, the trace metal clean technique was used to collect seawater samples from the upper 700 m of the water column using 5 L acid-cleaned Teflon-coated external-spring “Niskin-type” bottles (Ocean Test Equipment) on a powder‐coated trace metal clean rosette (Sea‐Bird Electronics). After collection, seawater was filtered through acid-washed 47 mm diameter 0.2 μm Supor polyethersulfone filters (Pall) and collected into acid-washed 1L LDPE bottles (Nalgene).

Seawater samples were acidified to pH 2 with concentrated, distilled HCl (0.1% v/v) in the clean lab at University of Southern California (USC). After acidification, seawater samples were stored for three months before trace metal analysis.

Metal concentration analyses were identical to those used in Hawco et al. (2020). For each sample, 15 mL seawater was transferred to a 15 mL tube, then 50 uL of an isotope spike (containing 57Fe, 62Ni, 65Cu, 67Zn, 207Pb, and 110Cd) was added to the 15 mL tube and thoroughly mixed with the sample. The samples would then sit overnight before they were preconcentrated by seaFAST system. The seaFAST system helped to preconcentrate the seawater samples and remove the salt matrix. About 10 mL of seawater was injected through the Nobias PA-1 column of seaFAST and 0.5 mL eluent (1M HNO3 containing 1 ppb In) was used to elute trace metals for concentration measurement. The trace metal concentrations were then measured by an Element 2 single collector sector field ICP-MS. Concentrations of Cd and Ni were derived by using an isotope dilution method. Concentrations of Mn and LREEs (La, Ce, Pr, and Nd) were quantified relative to the 10 ppb standard and corrected for incomplete sample loading using the recovery of Ni (Hawco et al., 2020). Trace metal concentrations were converted from nmol/L (or pmol/L) to nmol/kg (or pmol/kg) using a seawater density of 1.025 kg/L.

Researcher processing notes:

The data reduction algorithms are identical to those used in Hawco et. al. (2020), coded in Excel 2016.

BCO-DMO processing notes:

• Converted date format from %m/%d/%y to %Y-%m-%d
• Added a ISO formatted DateTime column

 

Coverage: 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.