Table of Contents

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

Seawater and suspended particles were collected 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-cleaned 47 mm-diameter 0.2 μm Supor polyethersulfone filters (Pall) into acid-washed 1 L low density polyethylene bottles (Nalgene). The filters with suspended particles (>0.2 μm) were stored in acid-washed 50 mL polyethylene centrifuge tubes. Size-fractionated marine particles were also collected at station P1 using a McLane pump (McLane Research Laboratories, Falmouth, MA, USA) equipped with a 4 mm mesh screen, a 142 mm-diameter Sefar polyester mesh prefilter (51 μm pore size) and a 142 mm-diameter 0.8 μm Pall Supor polyethersulfone filter at each sampling depth. All filters and filter holders were acid leached before use based.

Seawater samples (1L) were acidified to pH = 1.8 with 1 mL concentrated distilled HCl and added with 1 mL 30% H2O2, and left for over 1 month. For particulate samples, each 47 and 142 mm-diameter filter was digested by placing the filter in a 25 mL acid-washed Teflon vial containing 5 mL of 8 M HNO₃. HF was not used to digest the samples in order to minimize the decomposition of lithogenic materials. Samples were digested on a hot plate at 120°C for 12 h. After digestion, the filters were taken out, placed into acid-washed 15 mL centrifuge tubes and rinsed with 5 mL Milli-Q water. The Milli-Q water was transferred into the same digestion vials. The digested solution was then heated on a hot plate at 80°C in a trace-metal clean hood to nearly dry. Digested and dried particle samples were re-dissolved by adding 10 mL of 4 M HNO₃, transferred into acid-washed 15 mL centrifuge tubes, and then centrifuged at 2000 rpm for 5 mins to segregate insoluble particles and filter debris.

Seawater and particulate samples were then amended with double isotope spikes of 61Ni and 62Ni, 57Fe and 58Fe, 64Zn and 67Zn, and 110Cd and 112Cd in a spike-to-sample ratio of 2:1 for Fe, and 1:1 for Ni, Zn and Cd. Ni, Cd, Fe, Zn, and Cu in both types of samples were then extracted using Nobias PA1 resin. Metals of the seawater and particle extract were further separated and purified for isotopic analysis via anion exchange chromatography with AG-MP1 resin. As Fe, Zn and Cd samples were ready for analysis after the treatment, Cu required to be purified again by AG-MP1 resin and Ni required additional purification by Nobias PA-1 resin, followed by another separation using AG-MP1 to remove contaminants. Detailed information of the whole purification steps is described in Yang et al. (2020).

Isotopic measurements were performed using a multi-collector ICP-MS (Thermo Neptune) with a desolvator nebulizer as the sample introduction system (ESI Apex- IR), and the concentrations were determined using an high-resolution ICP-MS (Thermo Element 2) with a PC3 desolvation system.

Data processing was done using Excel 2016 with home-made data reduction algorithms. The data reduction algorithms were modified from Rudge et al. (2009).

BCO-DMO Processing:
- replaced "N/A" with "nd" to indicate "no data";
- added date-time field in ISO8601 format;
- renamed fields to comply with BCO-DMO naming conventions.

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.